How reliable a 22 year old moped can be is open to debate. Alot depends on the treatment it has received up to this point. In general the Vespa is a good bet as they are of a simple design and parts are still available from vespasupershop.com.
I bought my Vespa Grande 4 years ago with the intention of getting it running and selling it. Started using it and found it was much fun for the price. now I won't sell it!