I just went through this myself. The boat that I purchased had a totally illegal propane system.
I found that retrofitting a boat for a proper propane system was terribly expensive. I already had an almost new LPG stove but when I added up the cost of doing the system corrently I could buy a new stove for less. If you do the system right, you are buying a long list of expensive items with a few not so expensive items thrown in: Tank (go aluminum), shut off/connector, servo-valve (solinoid), properly made up gas lines, a sniffer/shut off switch/and carbon monoxide alarm, either a manufactured pre-fab tank storage unit or else building a proper storage well (sealed air tight with a hatch that opens directly to the sky {rather than ino a locker)with a drain at the bottom and a vent at the top; Both the drain and the vent opening over the side above the heeled waterline and no electrical connections within the container), tank hold downs,and a bilge blower system.
Given that I have seen way far more boats that burned or blew up because they had propane and a diesel than I have seen gasoline engined sailboats catch fire or burn, and that like most people I would be hesitant to buy a boat with a gas engine, it seemed obvious to me that LPG was a really bad idea.
I ended up deciding to install an Origo 600 non-pressure alcohol stove/oven. (Defender Marine has the best price.) The Origo is the safest of stoves that you can buy with almost no moving parts. Alcohol is easy to store.
Origo''s do not have the dangers and poor performance of the older pressurized alcohol stoves. The Origo burners have nearly the same heat output as a Marine LPG burner and more than twice the heat output of a normal pressure alcohol burner.
I have used an Origo on my last boat for over 13 years and they are really wonderful to own. I have spent too much time in my life searching for small propane leaks, to ever want propane on a boat of mine. Beside installing the Origo was way less expensive than putting in a proper LPG system.
Another really neat system (especially if you are going offshore) is the Wallas Stove and Oven. These burn diesel fuel in sealed compartments (no oil burning smell) so you only carry diesel fuel onboard. These are really well built units and although quite expensive are still way safer and well less expensive than installing an LPG system.
Jeff
Just my two cents here.