What engine is in it? Be sure to check for good compression and for leaks. Does it come with any accessories? If so, what? Does the car have a servo? What kind and model? What is the over all condition? How do the bearings feel? Is the suspension and steering tight? I'm guessing you don't have the answers to these questions? Well, prob not ALL the answers anyways!
Whenever buying used, be THOROUGH! Ask questions. How old is it? Was it races or just bashed, or both?( While I am against age discrimination of Any kind, I believe the age of the owner can sometimes tell about the kit as well. Usually... USUALLY younger owners are learning and can be harder on cars. Also, they don't typically have as much money to spend on their cars. This is something I learned about buying a used 1:1 car. I feel it transfers well to RC. IN GENERAL!!!) Any engine mods or repairs done? How does the air cleaner look? What fuel was used? 20%-30%? Check the condition of the shocks. Check to be sure the shafts aren't bent. Are they leaking? How does the shock oil look? Is it dirty? (good indication of poor maintenance) if you can check it, how does the line in the trans look? Are there leaks there? If there is a body, how does it look? Does it look like it spent a lot of time in its roof? Condition of shock towers for grinding. Also, the chassis itself... Is it bent? How does the underside look? Is it ground down badly?
What has been upgraded on it? What brand was used? RPM A-arms for example...
Wow! Lots of questions!
These are things I would be concerned with as when they are all out together, it creates a tail of use, abuse, maintenance etc... I would ask away as well as any others you have or that the ppl here add on to... Or maybe even suggest against. Keep in mind, it's unlikely they have a long line of people trying to buy this. Nitro isn't the fastest seller in a lot of cases. I would take this info and compare it to other used similar kits that you find on line... Search them out tonight and think about the prices vs conditions. Keep in mind, you are either buying someone else's baby, or their nightmare!( Some of these things may not apply to this specific kit. I don't own one so there may not be 'trans fluid' for instance.)
Whatever you find, let us know and we will try to give best suggestions! Good luck!!!
Oh, as far as the other questions... I don't ever get into electric conversions but there are a few who have with great luck! If
@olds97_lss Is around, he can tell you more about his adventures with a Jato...
As far as a different kit to check out, I really enjoy my Revo. Its a great kit with lots of parts availability. Its extremely versitile and is great for letting the kids bash with as its tough as nails! A monster 4x4 would be my next step unless you're really stuck on buggies. If you're on road a lot, a rally might be another option. I don't own one but have talked with a few owners who really like them! 4x4 has always been good to me... More expensive to own and maintain but they run everywhere!
If you're thinking about racing, that's a whole other story. Say so if you are and I will try to help you thru all that.
yet another purchase then
As for the remaining screw... Is it possible to grab the screw from underneath with some vice-grips? If the piece it is stuck in is metal, heating it will help to remove it but you need to be sure you can get a good hold of it so when it's hot you can move it. If you do go with heat, I would remove the entire part from the chassis before heat is applied. Take care to heat evenly and gently. In other words... Dont use an acetelyn torch unless you know how to do it. 
. Will keep you posted.
