OK, Paul... I think you know I'm a big fan of the OMs.
My vote for an affordable version would be the OM -1n. One of the OM-3s would be fabulous, but the price would shock you.
Several members have pointed out that the lens used will make a huge difference. Another factor is the focusing screen, of which there was a large selection. Most of the versions differed in the type of focusing aid - split screen, microprism, etc.
But the biggest difference was in the series of the screen. The earlier cameras all came with some style of the series 1 screen. With the advent of the OM-3 and 4 cameras - or thereabouts - came the series 2 screen, a much brighter animal.
Two of my OMs have bright screens installed. One has an Olympus 2-13 (I think) the other has a Beattie Brightscreen - they are equally brilliant.
Here is my favorite OM-1n:
http://www.rangefinderforum.com/photopost/showphoto.php?photo=128811&ppuser=20714
Incidentally - there are three variation of the OM-1:
Original OM-1
OM-1 MD added the socket for motor winders and drives - they made both.
OM-1n was the last in the line with numerous improvements, many of which have bee already mentioned.
For more information, check out John Hermanson's OM site. He is an ex-factory tech and the resident USA expert and ace rebuilder.
http://www.zuiko.com/
John has reworked all of my current cameras... and several others that I foolishly sold or traded.