unless the seller explicitely states that the camera has recently been through a general CLA (clean, lubricate, adjust) you almost certainly have to have it done yourself. There are several excellent Leica repair shops in Europe, they will set you back some hundred(s) euros but then you'll have a camera working for you the rest of your time! I bought an M3 in not too brilliant condition, had it serviced and although it doesn't have the looks of a museum piece, it's works a wonder and is just soooooo pleasant to handle. I think 600$ for an M2 is about right and the choice between M2 and M4, hmm.... I would select the cheaper one!
good luck