After wasting 2 years of my life working on forklifts (my boss was weird) I can honestly say from experience that up to around 4,000 pounds or so, go electric. Only thing that gets pricey with them is the battery, so make sure the battery is able to hold a charge before buying. Any more that around 4,000 pounds I would get a diesel preferably. Whats nice about a propane forklift is they always start right up no matter how cold and they don't give off much fumes. The down side to them is in humid/temperature changing weather, the distributor will typically get moisture under it and cause the engine to not start until you dry it out manually. Also with propane the fuel system parts tend to be a little pricey. Another thing with propane is if the forklift gets left out in the rain or gets washed, some are not very tolerant of water at all, the new CAT propane comes to mind, which are made by Mitsubishi. Diesel forklifts tend to be pretty bullet proof and forgiving on up keep and water. From experience only, the problems I see the most are hydraulic leaks, electrical problems from everyone hacking on everything, and a general lack of up keep. The new CAT forklifts are made by Mitsubishi and are identical, the old CAT's are made by Daewoo and are identical, the Clark forklifts are a real pain to work on due to design, very tight spots to get into. Any asian brand of forklift will use metric flare fittings on the engine/transmission so if work needs to be done the part has to be ordered, and some even use metric hydraulic fittings which NO ONE carries in stock!!!!!!!!!! Gasoline forklifts are a non stop pain to start and give off fumes like no tomorrow, and a forklift by use typically sits up for long times so the carburetor is always getting clogged.
Things to look for when buying...
-check the engine area, broken/cut wires are normal on older units along with lots of dirt, but any new shiny oily spot is a new leak that needs repair
-check the hydraulics, old hoses with falling off rubber from wear will probably need changing before too long, any hydraulic cylinder leak will require the cyclinder to be taken apart. Raise the forks with a load and let them sit in the air for a few minutes, if they drift down or forward then there is a leak internally that needs fixing
-check tires, on a "cushion tire" forklift the tires are solid rubber and can be driven until they are worn down, on a "air/foam tire" they have to be of decent condition or will need changing
-check under the dash/steering wheel area, this is where you will find most of the electrical hacks from everyone and their brother
-if possible, check the flywheel ring gear, if the ring gear teeth are damaged, the engine MUST be pulled in order to replace it. Typically the ring gear will always have 2 spots where the engine will always stop at and this is where most of the chipped teeth will occur.
-if possible, check the fuel tank if its a diesel or gasoline, the fuel tanks are built into the frame and over the years LOTS of rust and trash will collect and clog the fuel system, which requires the tank to be drained and washed out