When I first got my van I asked a friend who drives large vehicles for any tips. He had one.
"You know those big yellow arrows and suggested mph signs you see on tight turns? They're talking to YOU."
Funny. That's what the previous owner of my van said to me when I test drove it. Very true! Also, remember that you have an overdrive button on the gear shift as well as 2nd and 1st if it gets steeper. Use your transmission (gently) to save your brakes.
I'll second everything else said above, although beware that 'homebuilt' means a LOT of work and requires you to be handy. Two fundamental choices you need to make as you start your search:
1. 2wd or 4wd?
2. Gas or diesel?
Maybe a third is Ford or Sprinter?
Lots of information here on the forum about all of these. This will help you narrow your search parameters for used vehicles. Then, your search will be determined by what happens to come up for sale. Think about interior layouts and choice of tops (pop-top, Voyager, etc.), but be ready to be flexible if you are going used. Also, have your cash or financing ready to go quickly in case the perfect vehicle comes up.
Good luck. It's a lot of fun!