To the O.P.
My propane motor experience comes from Lehr outboards & forklift motors. The outboards work well as long as you are not in crazy cold temperatures. This is because they rely on the propane turning to a gaseous state before it reaches the mixing valve in the motor (carburetor type thing). In this case, the propane needs to "boil off" in the propane cylinder. Head space & temperature are the two key factors in making that happen. Most forklift motors don't have this problem, because they have vaporizers before the mixing valve. Most forklift motors are designed to take in the fuel as a liquid. The smaller forklifts that you see with the fuel tank standing straight up are the exception. Those little ones lack the vaporizer.
When you first fuel up a Lehr, it is best if you bleed the air out of the system before pulling the rope to start it. If you purge first, you can usually expect it to start on the first or second pull. If you don't purge, you can expect to get a sore arm.
Propane comes in 3 grades that I know of. They are 10, 5, & I. 10 is what you normally get for your gas grill. Technically, you are supposed to run grade 5 in a motor because it burns cleaner & doesn't leave plastic residue inside the motor. I've been using 10 for years with no problem. The I grade is a different story. That is intended as an industrial chemical feed stock & is likely to cause trouble in a motor.
Anytime you have propane on a boat, you are supposed to keep it out in the open or in a propane locker that vents overboard. In this way, the propane tank for your outboard is no different from the propane tank for your stove.
The composite propane tanks don't rust, but I think that they have a shorter time before they are required to be inspected, tested & marked with a certification date. The steel tanks, don't have the short test date requirement, but on a boat in salt water, they don't tend to hold up as well. Aluminum tanks are not a bad option if you can find them.
