We have a 36' x 36' (actually 24' x 36' with a 12' "lean to" on the east side), so close to what you're planning. It's a very simple design, open to the east, and a 12' double sliding door in the center of the main (24') section on the south side. There are 3-12' x 12' pens under the lean to (full 12' gates on the outside, 3' walk gates on the inside). Ours is all steel, but we had access to cheap drill stem. Wood works, too - I'd say whatever's cheapest/easiest.
In the northwest corner there's a 12' x 12' feed/tack room with a concrete floor, in the center on the north side a 12' x 12' concrete floored wash rack. For drainage, we sloped the concrete slightly to the north and let it drain off between the corrugations in the sheet metal. It actually works pretty good if you carry out the manure and try to keep it fairly clean. The ground slopes down to the north, and it's very sandy here, so it just waters the grass - doesn't even leave a puddle for long, even washing several calves.
All this leaves 24' x 24' of "open space". Even with all the stuff that stays there, it leaves enough room to park a pickup when a hail storm's coming.
We put in a big breaker box, with 6 - 110 outlets and 2 - 220's. Each outlet has it's own breaker, big enough to run a blower and a fan on each one. Lots of lights - we started with plain old lights, adding some flourescents over the washrack and where we clip.
We put up shade cloth around one of the 12' x 12' pens - even inside, it seems to help hold the cooler air. With a water cooler and a couple of fans, it stays cool enough to grow hair even in late summer (with a little elbow grease). We use cedar mulch in the pens - the sand swallows cedar fiber too fast.
There are lots of fancier show barns out there, but the only thing I really wish was that we'd have built it bigger. I have plans to add another lean to on the west side. 16' -18' x 36' would be plenty of room for a cooler and a clipping room...
