I have a friend who did exactly this several years ago for the purpose of transporting r/c helicopters and planes, and it turned out to be one of those things that is better in theory than reality. It was harder than he expected to gut the interior and the finished product wasn’t as spacious as expected.

Also, ex-emergency vehicles are mostly used-up by the time they are sold/auctioned, regardless of odo mileage. This applies to ambulances in particular because they spend LOTS of time idling and are very frequently driven hard and in short bursts.

I don’t know what your budget is, but you can easily find 8-10 year old E-250 cargo vans in good condition and mileage for less than 10k (these vans are fairly bulletproof and typically good for 300k+ miles). Ford Transit would probably also work great and can be had for similar money, but I don’t know if they are ultimately as durable as the Econolines.