155k is baby mileage for this car...just getting broken in...but it all depends on whether or not you like it. If you're worried about spending a few thousand on the car because it's got 155k, then you should probably just get rid of it.
If your trans indeed just has worn clutches, then the rebuild should be ~ $1000 from a reputable shop. My understanding of younger 525i trans configurations is lacking, but I'm pretty sure you don't have a ZF trans, but rather a GM with lifetime fluid? If so, it has to have that very specific, very expensive Shell fluid. If it's the ZF, then it's a very robust trans. The one in my car lasted to 220k+ with no apparent rebuild. Word of advice: if you want to keep it, don't drive it unless it's to a trans shop (assuming you've worked out the fluid level issues and that it ISN'T the problem). Even then, I'd have it towed were it mine.
Best, Whit