A few things...
The Super Chassis (upgraded chassis) is thicker and made out of a stronger/better aluminum (7075 in the upgraded chassis vs. 6065 for the stock). Well worth the money.
Don't bother with a front one-way. You're not racing and so far the only nitro car I've successfully ran with a front one way was the HPI Super Nitro...and it's a handful. For running around an open area, the stock gear diff with a really thick diff fluid (100,000 wt...no, not a typo) would be better as you retain braking ability.
Carbon Fiber is always helpful. The CF shock towers are stiffer thus giving you more "true" shock perfomance and the upperdeck helps the chassis resist flexing. (Don't really help CG all that much.)
Fiber disc brake is a must.
Engine, just about anything will work that's a small block. Rear exhaust will require a new header. The 3.0 powerplant isn't bad though (but if you're going to yank it and it's brand new let me know...may buy it off you...)
Bodies...200mm is "Wide" and 190mm is "Narrow" All Nitro TCs use 200 but some bodies will be slightly wider in the back for HPI's "vintage" offset tires for a more scale look.
And while we're talking tires, 31mm tires will not necessarily give you "more" traction. Traction comes from four things in a tire, contact patch size, weight on the tire, rubber compound, and tread design. Most 31mm tires are "sport" tires and their rubber compounds are designed to be hard enough to last some time but soft enough to give some traction. Most 24mm tires are race oriented so they provide maximum traction through a soft compound of rubber but at a cost in tire life.
When TCs first came out you had 26mm and 31mm tires. 31mm tires provided LESS traction since the cars weighed so little and the overall PSI of pressure put down on the wider tire was too little. 31mm tires all but disappeared. Then someone thought if 26 was better than 31, then 21mm tires would be better than 26. They were right (mainly because of the reduced weight) but cars started looking like they were riding around on 10-speed bike tires so ROAR mandated minimum width of tires would be 24mm.
I use to race my RS4 with 24mm tires in HPI's "R" compound series (20R, 23R, 27R, 33R, and 35R) and they worked much better than the harder compound 26mm tires. So if you're about performance, and run on clean, smooth pavement, look at the 24mm racing tires. If you want that vintage look, race on dirty parking lot surfaces, or will race that kind of body, the Vintage tires are perfect.
If the new engine is pull start, that will work over the roto start. I KNOW so many people are against pull starts because of "how hard" the engine is to start with them but I'm going to let you in on a little secret. If you tune the engine right, they'll start on a few pulls when cold and 1 or 2 when warmed up. Pull starts are great motivator because the pain in your arm will let you know you don't have it tuned right. LOL
HPI shocks...definatly go with the ones with the threaded shock bodies. All my cars have these and that's the way every shock should be made. Makes for easy ride height and tweak adjustments.
That's all I can think of for now. Have fun!