Personally speaking here, when it comes to rust on a vehicle where structure points are affected (Door Pillars, Rocker Panels, Sub-Frame, Floor Panels) I don't bother with it. Once rust has started in those areas, then you not only have to deal with getting rid of the rust, you also have to deal reinforcing that affected structure point which can be difficult to do. Underneath any vehicle is a problem area for rust and the first place it usually starts.
As for LPG I have had it on an Older GMC van (4.3 V6) and I personally didn't care for. If it already has it installed, then fine. Here where I live, the LPG conversion and fuel is way over priced and is just as expensive to run as regular fuel when you factor in the difference of fuel burned between each fuel type (lpg and gas). At one time (long ago) it was worth the conversion and the cost benefits from it, but the propane companies (around here) figured they were big time players like the gas companies and raised their prices to compete with them. And this is a bi product fuel that they just used to Burn off into the atmosphere.
The LPG mileage will drop by 10-20%. It depends on what system you have that will determine the mileage you get. Their aren't as many companies making them now and each have their own design for the LPG set-up. Some systems are more reliable than others. I can't give any names of the reliable systems because they have changed so much over the years. Maybe some members here that has the conversion can chime in, or you can do a google search to see if others have discussed this topic (LPG conversion reviews).
As for the vans themselves, they are very reliable overall as long as they are maintained properly and a good name brand used (AC Delco) for parts that need replacing. Hope this helps and I'm sure others will chime in with their own opinions with the questions you asked.