Quick response as I need to shoot:
Suspension: Stock suspension is not hard at all IMO. But it is subjective I guess. Finding stock suspension maybe more tricky than not though.
Room in the back: Don't know the difference, but there is F all room in the back of the supra. Next to pointless the back seats.
Roof: yes, the roof stores in the boot. You'll have no room for anything else in there while the roof is stowed. As for easy or removal, the roof is light, but I suspect 300ZX will be easier due to it being in two pieces. Not sure how 300 roof is attached but, there's 4 bolts (one in each corner) that needs to be tightened / loosened on the Aerotop (supra).