Have had both kits along with many other Tamiya cars.
1. Stay away from the FlySky radios. Just my own opinion but I tried one and it turned out to be a pain in the butt.
2. The 04SS can be changed to a regular wheelbase. The rear arms are different. Keep in mind the 04 cars are discontinued and parts can be tricky to find.
Yes, the 04 is a regular 185-190mm width chassis so any 185-190mm body will work BUT...the SS changes that so only certain short wheelbase bodies will work. Good luck with that !
The M06 is a M chassis so only M chassis bodies will work. The Alfa Gullieta will not fit the 04SS.
Lipos will work with the M06. 3000mah will work but you must make sure the case has the rounded edges. Gens Ace packs or even Reedy Wolfpacks should work. Check out the M06 thread for more help.