a slab is nice, but definetey not needed if you want to skip it.
i built my "shed" this winter.it is 10x12. fully insulated and also lined with osb. so its heavy. my floor is 2x8 16" on center. i park 1 harley, 2 dirt bikes, and 1 atv inside it. no problem. you dont need to go that heavy on the floor either if you arent parking heavy items in it.
a concrete truck will have to be able to get to with in say....12-16 feet...i am guessing. i think they usually only come with couple chutes. now if you are rich you could call out a pump truck and put the mud right where you want it...it make it soooo easy. but expensive.

