Well..i have recently got a 1200 with 3.0 roms...and os 3.5 working perfectly!
How is possible you ask? Well, this machine has a Blizzard 1230 accelerator, and a patch program called blizzkick, which makes the machine boots twice, but at the end of the day, i see os 3.5 running here...
Im not sure if blizzkick works only if you have a blizzard accel, but maybe a research on Aminet could show you if other programs do the little thing as well.
Anyway, i would also recommend you that before you buy the 3.1 roms, go for an accelerator!!
Regards!
Sebastian :-D