I bought an Isolette III with the Prontor shutter and Apotar lens a few years ago from Jurgen. I like it, basic but solid. I haven't used one with a Solinar, but since I mostly shoot at f/8 or smaller with it, I have my doubts if I'd see a substantial difference.
I have to say, if you compare with Jurgen's prices, $175 is pretty good for an Isolette III with a Solinar - assuming it's already had a CLA and/or new bellows. If not, and it needs one or both, probably not such a great idea.
If it was me, funds were limited, and I didn't know what to expect from a folder, I'd say go for the cheapest option (except you have to estimate distance or get either an accessory rangefinder or a laser tape measure or something like that), and work up from there eventually. This assumes, of course, that you can fondle the camera before purchase - maybe open it and shine a small flashlight from inside the bellows to check for leaks. I wouldn't worry so much about Apotar vs. Solinar, frankly.