Simple reason(s) for the above are:
(1) Size of the engine;
(2) Newer models (ie. M135i); etc.
Yes they are fairly affordable (compared to other cars that fall within the same characteristics bracket).
Not alot of known issues AFAIK with the exclusion of the normal service parts.
Depending on the mileage and overall condition (and maybe pics) we will be in a better position to advise you