The largest that will fit in the serpent is a small block. An OS 18 is still considered a small block but will disrupt the cg and design of the 710. If you plan to race 1/10 touring, plan on having some restrictions on the car unless you run it in the open/outlaw class. Meaning, 3 port .12 engine (ROAR/EFRA Legal).
I also run a Mugen MTX-4 and an Xray T2. The T2 is electric and the same engine constraints/size apply to the Mugen.
With these cars, and if you do plan to race them, you will need a few basic setup tools to get them performing correctly. ie.: setup station and tweak station. You can run the foams on them at full size or true them down to what you will best perform with via a tire truer.
If you just want the best of both worlds and be able to mess around and race when you want, the Nitro TC3 is your best bet. This car allows bashing and/or racing. Best bang for the buck since with any of the three cars mentioned above will cost anywhere from $350 (serpent 710) to $425 (Mugen MTX-4). And that's just the kit. Extra cost will be determined by engine, pipe, electronics and other odds and ends.
BTW, Lola bodies are only allowed in outlaw class. Everything else will require you to use a 4door touring car body.