Maximum vessel size

I currently do 5 gallon batches, AG and am looking to build a little brew sled for myself. For reasons of simplicity and cost I'd rather use kegs than pots for this setup but am wondering if there is a maximum size preference for this type of thing. Specifically, there has got to be minimum grain bed depth for mashing. Would a five gallon grain bill be too thin in a keg? Additionally, if I cut the top third or so of the keg off would that affect it's rigidity? Would I be better off with 1/4 barrel kegs.

It's my understanding that the need for a significant grainbed depth depends on your sparge method. When fly sparging I think you want a deeper grain bed for better efficiency, but with batch sparging it doesn't matter so much as long as you get a good filter through the husks to keep the grain out of the boil pot.

I'm sure someone will correct me if I'm wrong.

Just go with the kegs, and if you're making a beer that has too shallow of a grainbed depth, just batch sparge. You'll always regret having too little capacity more than having too much.