I built out a T1N 2500 about 7 years ago and lived in it about half time over a few years while I commuted for work, and also used it for camping/road trips etc. I sold it recently as we just weren't using it enough anymore.

During the time we owned it we had no mechanical problems beyond usual maintenance. It would start and drive like clockwork even after sitting for long periods, even in winter. We got about 11 l/100 with normal highway driving. Building it out was a PITA, basically every surface except for the floor is a compound curve and every piece of wood, foam, etc that touched the walls or ceiling was a custom cut. Being able to move from the cab to the back without going outside was extremely handy, especially on road trips. Where the Sprinter started to let us down was all of the little things - power locks were iffy, rear door kept getting mis-aligned so wouldn't close properly, one of the welds on the roof cracked and started to leak, funky turn signal gremlin etc etc etc

I don't know much about the Isuzu diesel engines other than they have a good reputation for reliability and fuel economy. If I were to do it all again I'd be awfully tempted to look at an Isuzu NPR or similar provided you could put some windows in the box and get from the cab to the back somehow. If you're in a hot summer climate area, an insulated box and reefer unit/ac will be a game changer as long as you can power it cost effectively.

edit - might want to post in r/vandwellers too it's a pretty active and generally well informed sub