As with any vehicle without warranty, you're taking chances. The first step to take before purchasing any vehicle is exactly what you're doing now: recon.. or, research.
These forums provide you with a brilliant database to find known issues with the model you're researching. Take your time, read the threads and make notes.. copious amounts of notes.
Armed with knowledge, take the car to a BMW dealership and have them run the VIN so you can see the maintenance records and find out if the previous owner 'cared' for the car. Now you'll need to spend a few $ and have them go over the car with a fine-tooth comb: Are there any issues that they see. What maintenance is coming up?
As a previous poster stated, ensure you look at the vehicle condition: is it mint? It may only have 70k kms, but were they hard
70kms, or babied? How is the carpeting? Leather? Is it clean? Rock chips on the hood/side panels?
I did my research before I bought my 328xi - it has a year of factory warranty left and I spent $3k in extended warranty. For me, it is peace of mind knowing that should something go wrong, it'll be covered.
Don't forget, of course, that you are buying a BMW and that the maintenance will be more costly. Rather than taking it to the BMW dealership for maintenance (as there is no factory warranty left anyhow), talk to your local BMW club and see if they will give you the name of a reputable tech so you have a go-to guy for the things you can't / won't take care of yourself.
Hope it helps a bit