answer for 2#: In my opinion a-chassis is the best chassis to use, it is very customisable and easy to use.
link for the latest a-chassis: A-Chassis 6 C Version 1.5 - Roblox
answer for 3#: yes, with a custom plugin, you could implement a nitrous system into the a-chassis
nitro plugins: VG's NITROUS - Roblox AC6 One Shot NOS + Wheelie - Roblox [FIXED/UPDATED] Nos Purge AC6 [FE+] Read Desc - Roblox
note that some of these nitro plugins might not work with the 1.5 version and can onyl be installed on an older a-chassis version. (you choose which one you want dont install all of them)
to install the plugin, you gotta drag the script into the “plugins” folder. then u gotta drag the other stuff into the “body” folder. (this is just what im guessing I didn’t take a look at the models) Also, if you’re lucky, the plugin will come with an instruction script.
if you have questions just ask