Ghazan Haider wrote:

How much does fuel cost for a C152 for a 600km journey?

Depends on where you buy it. You'll need about 22.5 U.S. gallons.

Is it better to try and get an engine that can burn mogas?

You can get a mogas STC for the 152.

I've seen car engine conversions. Are their TBO really high as in cars?

Not usually.

Whats a good choice on a serious
budget? Or rather; whats the cheapest way to fly 600km or so with two
people? four people?

Probably the old Hershey-bar Cherokee (150 or 160hp). That's a pretty good two
person plus luggage aircraft. It'll burn more gas than the 152, but it'll
usually make that trip without a fuel stop. It'll carry four in a pinch, but you
may have to leave the tanks less than full. If you buy one of the later 180 hp
models, that's a true four-seater.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.