I think I was unlucky with my MK2, first time I've had an issue with a lens.
Yep just saw one on eBay go for £100 secondhand you can get a new Mark 2 for about £70I've never really seen the advantage of spending £120+ on a metal mount 50mm F1.8 when you can get a newer Canon 50mm F1.4 for not a lot more, that is sharper and goes to F1.4!
Ask yourself, does it matter if the mount is metal or plastic? Do you manual focus using the scale? If not, don't worry about it.