RUN from the Sprinters!!!! They got their good reputation from the European models. The North American models are horrible.
A Sprinter was my dream van. Then when I was ready to buy I was given the warnings from people who know them. Expensive oil changes, INSANE part prices, unreliable parts and computers, rust, rust and more rust.
I bought a Ford Transit and converted it. You may be able to use some parts for the conversion but I would never try for a slide. Why put a huge hole into something and add weight for the little more space. Yes a van is small but if you want it mainly for the driving then live with the less space.
I love my van and would never go back to a truck camper, nor would I go larger to a Class C.
The journey to a vanHere is some about the conversionThere is more in the blog on the above link.