Yaaaaaaay! Lazu~ I'm so happy for you!
I love that red top and skirt combo. So pretty. I think that style of petticoat could work. It depends on what shape you want to give the dressy. But I would suggest a black petticoat just in case a little peaks through the bottom of the skirt from time to time.
The main things to look out for are material and lace quality. Material is super important because the last thing you want is for your dream dress to show up in a shiny-costume material. So just make sure it's good cotton or polyester or something like that. Lace quality is very important to some lolitas, but not to others. Personally, to me it's all about the look of the lace. Dresses can be super gorgeous even if the lace on them isn't the best. So you just have to make a decision about that. Also, unfortunately, it's very hard to tell lace quality until you actually get to touch the dress.
For the shoes, I'd say pretty much anything black or red. I suggest a good black shoe, because that will probably be able to match a lot of your lolita outfits in the future. I haven't ordered from them yet, but I know that
Bodyline has a good reputation for quality low-cost lolita wear. They have some really pretty shoes.
Good luck! I'm sure you'll look pretty in whatever you wear. ^^
<3