Once you park a couple of vehicles inside a shop, and floor mount some stuff (bender, grinders, notchers, or, if rich, a lathe etc) I imagine a 30x36 would start filling up. I dunno, I won't go much bigger than that simply because I can't afford too... but I can see why people with coin build monster shops.
I'd like to see pics of your dads too. No reason why normal siding can't be used on them... its wood framed, after all. Thats one reason why I'd like to go with a wood framed building... my dad has experience working on wood framed stuff, and wood working tools are always cheaper than metal ones.
As far as heat goes, I'd probably just pick up an old wood stove on craiglist. Pretty simple to cut holes in a wall to route the exhaust right? I assume they make some sort of sheet metal flanges just for that purpose... but what do I know!
One thing I definitely need to do before I make an offer on a house that I'd like to put a shop on, is call my county permit people just to make sure that I actually
can build a pole building on the land I am looking at. Nuttin' would be more annoying that buying a house, moving in and only then realizing that you aren't allowed to build a shop for some weird zoning/ordinance reasons.

That would suck, hard.
j