im looking to purchase a bmw 525i and was wondering if there are any main concerns with this type of car regarding repairs/servicing?
its a 1990 525i BMW Manual

As with many European cars, old BMWs tend to have some reliability issues, whether it's electrical or mechanical. Other parts will fail simply due to age. The only cars that have less issues are Japanese cars. That's just a statistical fact.

Why are they expensive to fix? They are not. With the abundance of internet parts suppliers, all you need to do is either learn to do minor repairs yourself or use a trusted garage. But if the only place you know to go is the dealer, then you just need to look at their rates to know why.

Common items to look for are water pump, ball joints/control arm, a/c final stage resistor, various sensors, shock mounts, radiator expansion tank, nothing a "weekend home mechanic" couldn't handle.

Expensive cars are expensive to fix.