Welcome to the Xmods world. Where to start huh? I guess I'll start with batteries. I don't think you can get away with 8 cells, the most I have heard of was 6 and that was on the 1st gen cars. Some people use lithium ion's or lithium polymers, but that does require some modification to use. Personally I use rechargeable Ni-MH AAA's. I get goo power and about 20 to 25 minutes run time on them. There are faster motors out there than what RS sells, but to use them you need to upgrade the FET's. As far the chassis goes, I'm not to sure if there are really any out there for the Evo's yet, but I could be wrong. The steps for modifying it isn't too much different than the gen1's, just some differnt parts that's all.
Check out this thread. It has links to a bunch of websites that have parts and info.
http://www.tinyrc.com/forums/showthr...threadid=21856
Plus there is a search box down at the b ottom of te forum page that you can use to search for topics like Evo chassis upgrades. There is a pretty good amount of stuff in here from other members.
Hope this helps and good luck.