Last late-summer-into-fall I did 10 CornishX in a 4x6 indoor pen *plus* all-day access to a 4x14 outdoor bedding-over-cement roofed run. (That would scale up to 60 birds in a 12x12 stall *if* they had access to a whole lotta outdoors, that was wind-sheltered and had excellent footing). It worked fine, as I processed about half of them at 6 wks which allowed more room for the rest of 'em for the last few weeks til I processed them too. I would not try to put more than that in there, personally - I'd rather they have room to move around (mine had muscles in places that no store-bought chicken carcass has muscles. pretty cool actually) and I don't honeslty think I could keep things clean enough unless I started removing all the litter frequently and topping it up 3-4x per day, which would be too expensive and burdensome for my tastes.
The smell is really really different, btw, not like regular chickens - more like a large dog that has eaten something that disagreed terribly with its innards. You can't clean the pen like you can clean a horse stall - you would either have to shovel out ALL of the bedding approximately daily (ka-ching), or actually I just covered it with a thin layer of fresh shavings 1-2x a day. I am pretty sure the smell of 60 birds in an enclosed barn would be horrible, though, even with decent cleaning. Really.
you don't need equipment to process them yourself, especially if you can find a helper (possibly paid in chickens?). Just a sharp axe or knife, a stump or etc if you use an axe, and miscellaneous buckets and table and so forth that you probably already have. You can rig something to scald them then hand-pluck; or you cna dry-pluck; or you can just skin them. I didn't buy a *thing*, myself (tho certainly if you could find someone with a plucker who'd rent or lend it to you, that'd be great). So, maybe it is something to consider.
Good luck,
Pat