M50 +sampling

I'm looking to buy an M50- how hard is it to sample sounds from internet say and then input them into the board?

It's impossible; the M50 has no sampling whatsoever. If you want to go the Korg route, get an M3 instead, or go previous generation and get any Triton or TR variant. The M3 and most Tritons have built-in sampling (in some cases, you can add more memory to be able to sample more); the Triton LE (i.e. 'Light Edition') and TR need to be fitted with the EXB-SMPL board in order to do any sampling, though you don't need anything but memory to play back samples that were recorded elsewhere.

Alternately, there's software that will let you make your own Korg samples from audio files in various formats, though the name escapes me right now.

As for other brands of keyboards, I don't know how their sampling works.