a "good chassis" for me is a chassis that is strong and can be dropped from 3 feet w/o breaking
materials for such as chassis - i would recommend HDPE plastic. Just get it like 1/4" or maybe even 1/2" if you want it really strong
As of shape.... well it doesn't really matter , except a circle has no sharp edges , a cool shape that I use sometimes is an octagon ,lol
Tracks are best for offroad though an all wheel drive car would work fine also. Tracks are the hardest to make, so buy it readymade