Just a few answers to your questions:

Nitropowered92; Unless you're running on a large track, stick with the .21 sized engines. .28's are generally overkill for arena trucks. I own a Picco built Orion .28, and it is honestly too much motor for the truck. Plus, the engine has been a little picky to tune. Look at the budget engines from OS, Novarossi, and RB Concepts, and stay with the .21's.

DONUT33321; On my track, I'm running the diffs at 7/15/5, and the shocks with 32.5 wt Losi oil front and rear. The pistons are the stock SS kit delrin pieces, white up front and black in the rear. The springs are pink in front and yellow out back. PM me if you need more detail.

zjatoRad; The simple answer to your wheel questions is that you can adapt anything to anything. You just have to find the right hex hubs. The Proline wheels use 23mm hexes, so check with Dace and Kellum Machine Works for those. I wouldn't recommend using the Revo wheels since they using 14mm hexes, which are smaller that the stock 17mm hexes common to truggies. Unless you are running on pavement, I would switch out the Aggressors for something like a Proline Bow Tie or Crime Fighter.

If you are buying the Hellfire SS, just stock up on extra screws, bumper braces, and lower arms. Also, get a spare filter, since it's always nice to have a backup.
If you are buying the RTR, grab the afore mentioned parts and be prepared to replace the stock shock towers and hinge pin braces for something sturdier.

I would suggest getting a starter box and ditching the pull/roto start options. If that isn't an option, go with the roto.