Pasi, I think you should only change thing that are worn out or faulty, otherwise it's damned to become expensive...
Cambelts, tensioners, and waterpump (you kill two rabbits with one stone here, because it's placed in the same area as the belts, so it saves you some serious cash) are essencial to be changed... no messing around there
But for example... Fuel Pump! Why? Is it faulty? isn't your car running well? It has to be expensive! Mine costs 450 euros!
Brakes are essencial too, but only need swapping if worn or warped... What you need to do is change the brake fluid and check the rubber piping... If it looks old change them, they're not expensive (about 8 - 10 euros each) and will save you from bursting an old pipe, ending up without brakes...
Exhaust if rotten... keep it
HEHEHE It's great fun for turning on other car's alarms
All the Best