HOLLAND??? Man, are you going to have a long drive home. Here is a quick and painless 1966 GT Mustang primer.
All 66 GT's came with a 289 cubic inch engine with a 4 barrell carberator. Most all of these were the standard 225 HP version, but a few had the 271 HP HIPO engine (these cars are very desirable and cost much more).
Any 289 or 302 (5.0L) engine will physically fit into these cars without any modifications, as will the larger 351w. The engines are quite responsive to "tuning", and a new intake and carb are an excellent place to start, as would be adding headers. These changes are cheap, easy, and effective.
The manual transmissions in these cars are very reliable, with most problems related to age. The clutches and the shifter mechanisms tend to be the only real problems. You will find, however, that there were no 5 speeds. Most of these cars, even as GT's, came with 3 speeds with 4 speeds being very common. I never understood anybody buying a GT with a 3 speed, but I've been told it was pretty common due to cheaper insurance premiums. The 5 speed is a very common swap and I can vouch for it. It is worth the time and effort in my oppinion. I don't think I could ever put the 3 speed back in mine.
I can't help with the LPG--I'm not sure what it is exactly, and would hate to steer you wrong on the subject.
Fuel consumption? Oh, I guess you don't call it MILEAGE over there do you?
These cars will get about 14-16 MILES PER GALLON. I could do the math, but I'm too tired to try right now. I guess you would be looking for km per liter?? Don't worry, it's not good.....it's right in there with most mini-vans.
Well, now that I've done my part to confuse foreign relations, I will bid thee farewell and wish you good luck in your hunting.