Yeah, definitely get it to a trustworthy BMW mechanic and have it checked out if you're serious about the car. The engine is very reliable. Make sure you get all service records. One critical thing that should be done on the 2.5L engine is to have the time belt changed at every 60,000Mi. So a car with 180,000Mi. should have it changed three times. Or if it hasn't been changed, prepare to spend about $200-250 for the job at most BMW mechanic (one word of advice: always take your car to a good BMW mechanic. There are some mechanics out there waiting to stick you up just because you have a BMW).
When you take your car in, have the mechanic pay close attention to the following: Water pump, fuel pump(mine went at 110,000Mi), compression (each cylinder should be at ~9-9.5), AC compressor, and check the valves for excessive noises. Also ask him/her to check the suspension.
Once you get your car: Take it to the same mech. and have all vital fluids drained and replaced. Engine oil, transmission,brake fluid, power steering (especially if it has turned black...normal power steering should be clear or red, depending on the make) radiator, and rear differential.
If everthing checks out, it will most likely be a wise purchase. The only gripe I have about the car is that it eats up quite a bit of gas for a car this size. This is due to the high-revving nature of the engine, for the most part.