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Basics of d20 game system using D&D 3rd ed specifics

Basic Stats and Modifiers
Basic character stats are used to modify most other things. They generally range from 3-18 each with a modifier from -5 to +5 (see table below).

STR - Strength: modifies melee attacks, damage, grappling, and some skills
DEX - Dexterity: modifies ranged attacks, AC, reflex saves, and some skills
CON - Constitution: modifies HP, fortitude saves, and some skills
INT - Intelligence: modifies some skills
WIS - Wisdom: modifies willpower saves, and some skills
CHA - Charisma: modifies some skills

1 = -5
2 = -4
3 = -4
4 = -3
5 = -3
6 = -2
7 = -2
8 = -1
9 = -1
10 = +0
11 = +0
12 = +1
13 = +1
14 = +2
15 = +2
16 = +3
17 = +3
18 = +4
19 = +4
20 = +5

Hit Points

The pool of damage a character can take is called their Hit Points (HP). It's based on their character class (in this case their model type) which determines their Hit Dice (d4 - d12), plus their CON modifier, times their level.

HP = (HD + CON mod) * Level

For example: A character of a model type with decent Hit Dice (d6) and a CON of 16 at 4th level: 44 = (6 + 3) * 4

Armor Class

The number an opponent must roll to hit a character is called their Armor Class (AC). It starts at a base of 10 plus their DEX modifier, plus their armor's bonus, and possibly some other things like a racial bonus or shield.

AC = 10 + DEX mod + Armor + Other

For example: A character with a DEX of 16, armor with a +5 bonus, and a racial bonus of +2 for being thick skinned. 20 = 10 + 3 + 5 + 2

Attack Bonuses

The number a character pits against an opponent's AC in an attack is based on a d20 roll plus their character class's (in this case their model type) Base Attack Bonus (BAB) progression (by level) plus the appropriate modifier for the attack and any other adjustments for the weapon, conditions of the attack, or special skills. The modifier for melee attacks is STR, for ranged is DEX.

MAB = d20 + BAB + STR

RAB = d20 + BAB + DEX

Fortitude Save

A character's resistance to disease, system shock, and toxins is called their Fortitude Save (Fort). It's based on their character class (in this case their model type) which determines their Fort base progression (by level), plus their CON modifier. When a character encounters a threat to their fortitude they roll a d20 and add their Fort save to it then compare against the threat's Difficulty Class (DC) which is a fairly arbitrarily set number by the GM.

Reflex Save

A character's ability to dodge and maintain balance is called their Reflex Save (Ref). It's based on their character class (in this case their model type) which determines their Ref base progression (by level), plus their DEX modifier. When a character encounters a threat to their reflexes they roll a d20 and add their Ref save to it then compare against the threat's Difficulty Class (DC) which is a fairly arbitrarily set number by the GM.

Willpower Save

A character's resistance to mind altering effects is called their Willpower Save (Will). It's based on their character class (in this case their model type) which determines their Will base progression (by level), plus their WIS modifier. When a character encounters a threat to their willpower they roll a d20 and add their Will save to it then compare against the threat's Difficulty Class (DC) which is a fairly arbitrarily set number by the GM.

Skills

Skill modifiers are based on the appropriate modifier for the skill plus any ranks the character has in that skill, plus any other bonuses for said skill. Skill ranks are purchased from a skill point pool which is determined by character class and level. When a character uses a skill they roll a d20 and add that skill's modifier to it then compare against the attempt's Difficulty Class (DC) which is a fairly arbitrarily set number by the GM unless it is an opposed skill such as stealth in which case the DC is the opposing character's opposing skill roll.

A few possibly applicable skills:

Stealth (DEX) - moving silently and hiding
Spot (WIS) - observational skills, used to oppose stealth
Search (INT) - ability to find things
Decipher Scripts (INT) - ability to decipher codes
Disable Device (DEX) - ability to disarm traps, alarms, bombs, etc

The interior space is mostly composed of isolated cavernous spaces, ether gaps between junk or large interior spaces of ships, connected mostly be smaller tunnels and old corridors, again from the ships that compose the hulk. All of this twisted, wrecked, and warped, as well as coated with decay and in some places built over with a material resembling that of an insect hive. In the less destroyed regions it can feel like walking through a derelict that's been floating through the void, untouched and dusty. Most of the surviving architecture is from Imperium vessels so the corridors are much larger than you would find on a Tau ship. Some Eldar constructs are present but they are much, much older than anything else, so they are mostly located in the center of the Hulk. Some of the component structures are still functional, or at least partly functional, but as most 40K races dislike computer intelligence none of it is active beyond the most basic measures.

Plot direction:
Hulk exits warp in an outer edge Tau occupied hot combat system, they've never seen anything like it but it's sending out an Imperium transponder and has some obvious Tyranid components. High Command is a few days out of comm range so a mid level Ethereal and a science team convince the reigning military authorities to let them go check it out. During the entry some ancient defense system activates and fires on the small fleet, destroying a minority of it and forcing the rest to land early, a combination of fire from the hulk and defensive fire from the fleet caused the bay that they landed in to collapse, disabling the remaining ships. The largest of the remaining ship was left structurally sound, though it's hull was punctured in numerous places, One of the smaller ships was destroyed in the collapse, one suffered a massive breach and had to be sealed from the outside, the last suffered a core breach, not catastrophic but the ship's interior was rendered radioactive and had to be evacuated.

Shortly after all this the Hulk slipped back into the warp, drifting with them in tow to who knows where, so now the passengers don't just have to find their way off the hulk, they have to do so in a ship with FTL capability, but it only gets more complicated as they venture deeper into the Hulk as they find that they are not alone, and not the first Tau to step foot here.

Because of the story most characters will need to be Tau, I'm willing to discuss other races but they will have to be present on the hulk prior to the above landing or possibly come in later when the Hulk emerges from the void again; in both situations the character won't join the action until later on.

Having resident characters is definitely going to complicate the story, especially given that these races are so prone to "shoot them all and sort out the friendlies later", so I am weary of them. One thing I'm intending on that will dissuade them is that resident characters will all be solo, so you don't get the "extra lives" of having a squad. The would logically also have less equipment because they've been surviving there for longer, and their equipment could very well be damaged as well; whereas the Tau squad would have had fully functional and loaded gear just prior to landing.

The depth of the 'squadmates' is up to the player I guess, if you feel like you can write all of them in detail with developed personalities I won't stop you, but for players with less skill or time, I'll finish this sentence later.