Thanks for the helpful tips so far everyone. I know it is a general question, but it appears there are a few "rules of thumb" involved in the process.
@ Mancha - I'll keep that in mind, although it is a pretty slim vehicle...
@ Sangennaru - Yeah I have split those up in the design e.g. steering wheel, tyres etc.
I might as well show what I'm planning on building:

Well... something like that.
The main carriage will be one piece, perhaps with some details on the bottom to put on separately. Wheels will be separate, as will the steering. Metal rails may be a go, too.
Not sure wether resin or metal would be better, but hopefully I won't need to worry about that just yet.