Couple of things you can do first to make a better decision as to what you need as a minimum for a RODI set up.
1. Get a water report from your supplier, whoever that may be, city, well or whatever. If you're on city water, they should have that info available on their web site somewhere. They should have reports including where the source is and some of the minerals etc. that are in your tap water. It will also tell you if they use chlorine or chloramine to treat the water. This is important since if they use chloramine, you generally need two carbon stages before your RO membrane.
2. Get hold of a pressure guage (beg, borrow, buy if you have to) and find out what your house pressure is. Should be around 60 psi and if so, you're good to go. If it's much lower, you may need a booster pump. I've tweaked the PRV in my house to give me just over 80 psi.
3. Get hold of a handheld TDS meter (beg, borrow, buy if you have to) and check the TDS of your tap water. This info may be available in the report from your city, but it never hurts to physically check at your house to make sure. Knowing the TDS will also give you a better idea of what you're up against.
Post back with some of this info and RC members will be better able to advise further.
Some advice on the cheaper eBay products..... They generally have white plastic filter housings, so you can't see when your poly filter might need changing, Clear filter housings are more expensive, but worth it IMO. The cheapies may also have their DI stage laying on its side, this is less efficient than an upright DI stage.