well that's just the problem, the wiring is stock... it's probably fine but you can't hurt it by returning it to running 55w halogen bulbs... you can hurt it by running ballasts off it. HID ballasts run a short but intense jolt of thousands of watts to the capsules to ignite them on start-up, and they pull a massive load through the wiring to do this. it only lasts a few milliseconds, so while the stock wiring will get superheated and weakened by this, it's not known to cause an engine fire every time. i am positive that it has melted wires before though, i've seen pictures of it.
parts and costs depend... i got my projectors for 50 dollars without glass(which i have and may upgrade), but i still don't have a d2s hid setup... so if you get a new d2s kit that can be 150-200, then the projectors which can be 100-200, a relay harness which can cost 40 in parts or 60-100 premade, and then the labor which doesn't need to cost you anything beyond materials(a few nuts and bolts and a dremel for the projector fitting, and solder and shrink wrap for the harness). it can add up quick...
a vendor on here, lightwerkz, does the whole thing and charges a good fee, and also backs EVERYTHING up with a warranty, even the ballasts and bulbs; check him out by searching his name or going to his site, www.lightwerkz.net
but be prepared because pricing is competitive but professional-grade.
a lot of people pay that for the best results and the peace of mind of having it backed against any problems, but it really comes down to what you're looking for lighting wise and what your time is worth. i've made my own relay harness, and it was awesome but took me quite a few hours and nights.
honestly I think it's worth it to spend the money on the system that is designed to accept the lightsource and give the proper output... if that means staying with halogen... well i can see just fine with halogen, at least until i can afford the real deal HID optics.
anyway good luck and i hope things work out for the best. you might be just fine for years to come with another ballast and no relay, but then again you could melt your stock wiring next week... i wouldn't want to give you ill advice. i think it's worth the peace of mind to just run stock h7 halogen until you can do it right.