This is just my opinion however here goes;
1) It doesn't look like a HPI RS4 to me, HPI tend to print their logo on every part possible, such as the battery box etc. the other point would be on the RS4 the engine cooling head is black not silver, and I may get shot to pieces on this one, but I am fairly positive this is the car that was built piece by piece from a magazine, the engine look familiar but that could just be the engine, I am guessing but I am pretty sure that is not a HPI RS4.
2) As it is a 1/10th scale model any body will fit, there are a wide range available from your local model shop. It is your choice to go painted or unpainted (My preference is unpainted but that is just because I enjoy that side of the hobby) Nearly all bodies come without the holes for body posts, so it is a matter of lining the body up and marking where to make the hole for each body post and once very sure its all straight and it clears the wheels etc drill or "Ream" the holes to the correct size. There is a huge variety of types out there so take your time and choose something you will enjoy crunching up!! LOL
3) If you are using the same fuel as was used to "Run In" the vehicle nothing special is really necessary, however if you are using different fuel you need to repeat the process. In both cases I suggest after-run, again you can get it from your local hobby store who will advise you how to use it.
Looking from your user name, please if this is your entry into the RC world don't be put off, it is tremendous fun, you can charge round your local car park or even up and down your street gaining a huge amount of attention because these things go like rocket ships, you may fid yourself wanting a challenge, go racing, there are loads of clubs who can help you get the most out of the car. Tons of fun are to be had in so many different ways, so bear with it and enjoy. If you need anymore help, jump on here and ask or head to your local store and they will also be happy to help I am sure.
All The Best