Thinking about buying a utility trailer to get my lumber home from the
lumber yard.

You'll use it for a lot more than lumber if you get one. That's why I
bought mine, and that's what happened with me. (Obviously you and I are
exactly the same. ) My first trip with it was a camping trip.

I googled and found out that you can buy kits and build your own. Anyone
have experience doing this? Is it a PITA? Am I better off buying one
preassembled somewhere (used?)

About five years ago I had a preconceived notion that a trailer should cost
about $400 Cdn used when I bought one of those folding trailer kits from
Canadian Tire for about that amount. What you get for that money is wheels
and a frame, and some really lousy electrical stuff. I added nearly $200
worth of wood to make a lidded box on top before I could use it for
anything. I was in a really big hurry (long hours at work to make time for
a vacation) and couldn't wait around for the right deal. The first day of
my vacation was consumed building the box for the trailer.

If I had to do it over again I wouldn't buy one of those kits. Mine
struggled to bring home a load of topsoil. It's wheels are small so speed
is limited and it jack-knife's easily in reverse. I'd spend twice as much
money and get something with a welded frame next time. Maybe that's just
me, but this one has taken more time and money than I ever expected.

But I use it a lot, so every penny has been justified. YMMV.

- Owen -