i bought a used 87 astro Van and got it converted into propane for my work and that van is running strong after over a year 1/2 of work and intense driving and still driving strong even after over 389,000,00 KM. All there is into buying used van is to do your research, how many owners and service record of the vehicle. if you have none of those? go for a test drive and look for a leak(s) and wear or the drive train or have it inspected.
Good luck