Windows can help or hurt you. The feeling of openness is nice - I like the windows in my van because it's small, so I hate feeling like I'm locked in a tiny box. That said, windows are a thermal problem if you're trying to insulate. A sheet of reflectix cut to fit works wonders, but isn't as convenient as a curtain (look into thermal linings), or as efficient as closed-cell foam on a solid wall.
I started mine with all the interior intact, and just worked around it. (Changed to a reclining bed/bench seat, etc.). Slowly I've been removing bits of the plastic trim here and there in order to reclaim some of the lost volume behind and in order to make things more space efficient. I have maybe 75% of the original trim in place now, and will probably get down to about 50% when I'm done building cabinets, etc. (Front cabin will stay intact.)
As for that CL van - hard to tell from a few photos, you really need to get a mechanic to look at it. The mechanicals are all fairly simple and stout, so how long they'll survive is entirely a factor of how they were maintained. Will almost certainly need a certain amount of work on rubber parts (bushings, hoses, belts), and who knows beyond that.
The double side barn doors are cool, and will give you some flexibility in terms of door boxes and layout, but if you try to keep BOTH sides free to be opened, you'll potentially be sacrificing a lot of interior volume.
