I ran a VW Passat Combi on LPG for a few years. The car was a lardass anyway, so I don't think the LPG made much difference. Filling up half price was great, though.
You can pay more or less than that. A friend of mine paid 6000, and I've heard of places that go as low as 1500.
find a place with the best recommendations
I'm not sure if converting a small car with the engine less than 1.8-1.9 is worth it.
Depends on how young the car is and how long you plan to keep it. You can, of course, buy one with LPG conversion already done.
technical inspection is slightly bigger
Quite a bit bigger as I remember.