When to replace bushes?

I know from your other post that this refers to a 1997 E36 325tds.

The only bushing that would likely need to be replaced is the front control arm rear bushings.

Well, the upper shock mounts have to be shot too, but I do not believe for a minute that you have never replaced those yet... ;-)

The other bushings are all in the rear end and can last a bit longer. I'd only investigate those if you're having some sort of problems.

Why not bring it in to a trusted specialist and have them give it a once over. If you do not have a relationship with them yet, just tell them you are considering buying the car and want a pre-purchase inspection. That will prevent them from listing off a bunch of stuff that doesn't need replacing in hopes of getting the business.

-Fred W