So, I can only speak for having bought commercial doors. I looked at building my own, but it wasnt where I wanted to be.
Recommendations I read that concerned me:
Its good to double or triple the seals. Include drains to prevent water build up
Use at least 3/4" birch plywood to prevent warping (didnt leave me much thickness to insulate on walls that were 1.25" thick after insulation.)
use trailer specific door handles and hardware (I looked at using a normal door knob to save $$$)
The added time taken in the shop, when I started mine in April, and wanted to have it out by August
Add in that if you don't fully seal the wood of the doors, they can swell and shrink as weather changes, and it makes it hard to do (at least with my limited skills) and ensure a water tight door.
I bought mine. Foam core, so they are insulated. Windows were set up a bit in the door so it doesn't go below the mattress, and the price was reasonable.
I got mine form vintage technologies for $289 each
http://teardroptrailerparts.com/26x36_F ... _Door.htmlWhen I build my next tear, I think I'm doing the same. The aluminum trim around the door gets cold, the window in it can get cold (but its a single pane glass window, which is at least recessed so you don't generally roll against it) and the trim around the window gets cold. The "meat" of the door though doesn't.
Its been through torrential rains while driving down the road (kids called it the hurricane simulator) without leaks, and its stored outside in Michigan without issues (I do have it tarped right now to keep water out so I dont have a freeze/thaw effect on the seals, and to keep melting snow from coming over the fantastic fan vent in the roof)
I've had no reason to regret that decision.