I'd look at the petrol side first.
As I said, your fuel system could be gunked up as it sounds. That means the fuel pump won't flow fuel when powered up, which is why it stutters and dies when you squirt fuel into the throttle.. Gunked up fuel pumps can overheat and die if left powered up for tool long as they need fuel flow to keep them cool, especially the pump types that require a return fuel line to the tank.
As such, and depending on your interest, tools and skill level, I'd look at dropping the tank and cleaning the inside of the tank (be prepared to be shocked). Then flush the fuel lines and return line, replace the fuel filter and remove the injectors and clean them or have them cleaned professionally. Do that and it should by all accounts fire up.
Injectors can be cleaner as shown here..
If the injector is really clogged, soak it in isopropyl alcohol as it dissolves some fuel varnishes and then try the 9v battery trick
Then once the engine comes to life, you'd do other maintenance checks like ensuring all tank vent lines are unobstructed, the purge solenoid (if any) is working, vacuum pcv and other lines aren't perished, fuel pressure regulator is working, new oil and oil filter, new spark plugs and leads, new air filter, flush cooling system, flush auto fluid, etc... maybe new coil packs... definitely flush brake fluid and check brakes parts aren't now leaking due to rust... where do you want to stop
Then have someone licensed look at the LPG side and get it working/certified unless you don't want to keep it (but it will still cost to legally remove it so why not keep?).
Oh, and then don't leave cars sitting for so long unloved unless you've prepared them for long term storage (which is a big job itself)