I'll answer what I can and the others can hopefully fill the gaps.
Numbers, we get around teens upwards, I think the max we've had is 20, but that was before we bought the timing system. With winter inbound now numbers do drop. Spring time next year will be the true test
2WD - B4 is the most popular
4WD - Cat SX is the most popular (Phil runs a B44 well)
The more popular class was 2WD, but 4WD numbers have picked up. I think it's the time of year. I am planning on running my 2WD from Spring again
Track hasn't changed much, no big jumps so it's good racing, but not a car breaker.
Last Sunday it was monsoon like with lots of standing water. Hopefully tomorrow will be ok, just typical the weather is nice today. Come down and have a chat if it's not chucking it down?
Tyres - see Steve's post here -
http://www.oople.com/forums/showthread.php?t=33958
To get started I think B4 would be the cheapest route, the B44 is a nice machine but does cost a fair bit.
I am running a Cat SX LIPO with Tekin ESC and Losi 7.5 motor. 1 pack of Speed Passion 5300s (planning on getting another just to remove the pressure of having to worry about charging it, plus if I have a problem with it.)
I hope this helps.