So a 135mm L for £699 second hand or the 85mm mk1 second hand for £899?
I'm a huge fan of the 85 1.2. I've got the Mark 2 of that lens, but the biggest problem is the slow focusing.
The 135 I hear is a great lens, but the largest problem with that is distance required to shoot. In studio, unless you have a lot of room to manuever, it's a bit difficult to shoot with. Outdoors though, it should be fine!