And you'd have to mix many small batches to pour a large pad so would likely end up with a patchy and inferior pad. Order your cement delivered in a big truck. You might be able to save costs by having your own crew help. 6s not 2 x 4s for stall framing and any areas of the barn that horses will contact. 2 x 4s are just not strong enough to withstand horse force.
Planed boards are smoother and thinner.
Don't cut costs by not lining your stalls. An unlined metal barn is an invitation for some of the nastiest wounds, not only from a hoof punching a hole through the wall but from a pawing or rolling horse getting a hoof caught under the sharp bottom edge of the steel wall. Stalls should be lined at least four feet up from ground level. Two inch thick boards are best. All hardware, bolts, hinges, handles, latches and locks should be of the very sturdiest materials available.