As one of the few LPG users here it depends on how you view the economics.
I didn't want to add value, break even for me was 21,000 miles. My view was that the car (Volvo) is a solid car, no rust, runs the same today @ 140,000 miles as it did when we bought it at 62,000 miles and so we have no plans to change the car. The cost of conversion was about £1500 but I'd have expected to pay more for that to change the car to say a diesel.
My vehicle was converted at 120,000 miles and it has only just failed it's first MOT due to worn ball joint and split CV boot. Like I said, no plans to change a perfectly good car so for me the LPG conversion was worth it.
175,000 miles I think I'd tend to agree though, that's on the high side already but lets take mileage out of the equation, my questions would simply be:-
1. Is it planned to keep the car with or without conversion? What's the exterior/interior like, will it last the distance?
2. Is the car mechanically sound to the point where you're not expecting to be doing more major work, head gasket, timing chains, etc etc. Some people feel that any major mechanical work is a reason to punt the car rather than treat it as a cost of ownership compared with depreciation costs.
3. Is there an LPG filling station nearby? It can be annoying if you have to go a long way out of your way in your home area.
David.