I would suggest not losing sight of what you're after. A couple cars for you and your son to run. The kits discussed in here are awesome, for sure. But there have been a number of suggestions to get you guys going with good RTR stuff, that will have you guys enjoying the hobby. You did mention you're not mechanically inclined. So a good, jump in and get your feet wet with decent stuff kinda start will ease you into working on them. And as you can see, there's PLENTY of help here.
1. Don't listen to hobby shops, unless you get to know the folks running it and trust their opinion. You need to go in there knowing more than they do about what you're buying, if possible. If not, have questions ready. You did your research as best you could. See what else they know.
2. Buy a good charger NOW. Not later, after suffering through 3 or 4 mistakes. Stay away from smart battery technology. It's just not worth it.
3. Buy a quality metric hex driver set now. Not later after you've mangled screws in your kits.
4. A Good soldering gun/station is important. Learn to use it! It's not rocket science.