When I went to school in the centre of Germany all you could see in the teachers parking lot was one Beetle after the next and if there was something else it would have been a Type 3 or Kombi.
The Volkswagen was called the peoples-car for the simple reason it was economical to buy and run and it was reliable. No other company has ever sold more cars than the Beetle model did.
The aircooled Beetle Boxer motor was designed when Petrol was cheap which means it is not the most economical engine when you look at the petrol consumption but you get your money back when you get your service done or when buying Spare parts.
A 1600cc twin port Beetle motor will use about 10L of Petrol per 100km or you get about 10km out of 1L petrol. This of course is depending on your riding style.
The VW Gearbox is just as tuff and is used in many race-cars. It can handle more than 150HP with no problems. In the early days (unfortunately some still do the same now) Trike builders picked a gearbox from a car wreck and stuck it straight in the Trike. The problem is, the gearbox could have come from a 1200, 1300, 1500 car or even an old kombi. So the ratio is all over the place. I have seen Trikes doing 3600rpm at 100km/h which not only feels and sounds horrible but the motor wouldn't be too happy either. If the ratio is right the motor should do about 3000rpm or less at 100km/h.
I used to build beach buggies many moons ago and we just got gearboxes from the wreckers some lasted and worked ok and others didn't and the same goes for motors.
If you are not very technical minded and just want to cruise long distances with something you know won't let you down, then get a motor and gearbox built by someone you can trust. If you are a bit of a gambler and don't mind getting your hands dirty buy second hand motors and gearbox and go for it.