i bought a 2017 ram promaster 2500 136 high roof with 110k miles for $19k. it was literally the cheapest Ram van in the entire US on Autotrader. there were a few others close to that price but this was def the cheapest. so far so good on everything. no issues.
Transmission will mostly go out around 60-80K miles. I really want to buy one of these big vans but reliability is just not there. I do small maintenance work and HVAC as a side job. Been using Toyota Sienna vans for a while now. My last one made it to 260K miles with minimal problems.