"some of your other postings you seem not to like it that much"
No, I dont. Reasons:
1) not a true differential (noise)
2) max speed 9600 (due to above)
3) collisions all the time
4) a bitch to code (most implementations are not 100% correct)

"How do I check whether the bus is busy"
Connect Rx to int0

"J1708 would avoid bus contention"
NO, it does not AVOID bus contention it DETECTS bus contention.

"Have you had any other experiences more closely related to the RS485 limits, 32devices, 1200m, upto 10MBps (dependant on factors of course)"

Yes, with 485 at low speed I can go 4 km. The speed and distance are inversely proportional. Using 1/4 load transcieveres you can have yp to 128 units.

I have seen J1708 units contending getting into a fight (contention) that took too long to be resolved and thus led to failure.