Based on a little research, the sprinters aren’t exactly known for longevity, so there is a reason why high mileage ones don’t cost very much. Have you sat in the front of one? I don’t know though ... you might be small, so the reduced leg room might not be an issue for you.
For van style conversions, there are other platforms like the Ford that might be an option as well. I’d consider Ford Ecoboost as the engine. Good mileage but with a turbo, it’s also good in the mountains as well.
Also, from all I’ve heard, most initial slide designs fail miserably. You have to dramatically increase the strength of the side walls just to carry the slide weight, and make sure you have a good slide mechanism as well. Not saying you wouldn’t eventually get it right, but it’s probably harder than any other part of the build.
In any case, I’d figure out what interior you want to build and see what it fits in.