There's no single "correct" answer to your question, but there are many radio systems that will work.
Taking your first point - up to date transmitter.
This is really a question of what RF transmission system to choose & the answer is pretty much universally 2.4GHz these days for convenience, isolation & (mostly) freedom from interference. I suggest looking at bigger brand names & look for a system with fairly low cost receivers if you expect to purchase several. OrangeRX comes to mind, Spektrum compatible but without the price premium, a wide range of receivers at budget friendly prices. Swapping receivers via Velcro is feasible (indeed I have done it myself) but it soon becomes tiresome.
The multiple channels (3+) question is more tricky.
If the majority of your cars are regular 2ch & only one or two needing more functions, then I would seriously consider a dedicated multi-channel system for those 3ch+ vehicles & a really nice 2ch set for the others. Again look at a well supported brand, but try not to hamper your choices. The really tricky point is that anything more than 3ch will be a pair of X-Y sticks since this is what aircraft use. If you want a 4ch wheel system, the only one I'm aware of is the Etronix, but if you're considering 3 speeds or trucks, the 4ch stick radio is the only sensible way to go IMHO because of the H-gate selector for gears / throttle & still needing a steering axis.
There are many other factors to consider but this is enough to think about initially I'm sure.
