Can't help with pop-top suppliers, but can share some of my experiences with an '83 VW Westfalia. This was the first year that VW went to a watercooled engine. (4 cyl, 1.9 litre - kinda gutless on hills, but not bad at cruising speed. With a tail wind.) These early watercooled engines are notorious for leaky head gaskets and other coolant issues. My engine is at about 140,000 km (what's that, about 80,00 miles) and on it's last legs. A positive aspect to this is the people factor- I've met many of the tow truck drivers and VW repair folks in western Canada over the last few years.
The camper aspect is great. If you arrive at your destination, pull in reasonably level, pop the top, and you're done. None of the set-up pains I went through when I had a tent trailer. Propane stove, three-way fridge, water, storage. There's room to sleep four, but with luggage, instruments etc., two adults and a small child would be a tight fit.
If you are looking for a new version of the VW camper, expect to pay house-like prices. Even older vans seem to hold their value. Also, if you are used to the sound insulation provided by a hard-shell trailer, the VW, with the canvas sided pop-top is more tent-like.
Another thing if you end up with a VW camper - you are automatically and irrevocably enrolled in the "Cult of waving at other VW van drivers". I don't know if this happens with, say, Topaz drivers, but when one sees another VW van, one is obligated to acknowledge with a wave. Once, I neglected to do so, and was visited with a case of the guilt miseries for weeks. Corrollory to this - it was a VW westy driver that rescued us from the side of the road and drove us to the nearest town during one of our van's breakdowns. "We are family...."
The van is perfect for us from a camping aspect. We can keep the bedding, some clothes, non-perishable food stowed, so need only do a little packing before we hit the road. Too bad the thing is so unreliable engine-wise. I'm thinking about getting someone to drop a new engine in, but that's a few dollars that I'm not sure I can justify spending on a vehicle this old. I've actually considered doing something like you're thinking of- gutting the VW camping bits and installing them in a Chevy or Ford. But then, who would I wave at?