Regarding buying a 7D (and assuming you haven't bought it yet), here's an excellent article about buying 2nd hand:
http://www.dpreview.com/articles/4701677448/how-to-inspect-a-used-canon-eos-7d
He even specifies which firmware versions you're looking for.
There in nothing specific you need to look for regarding Magic Lantern. If the 7D passes all your other tests then it will be ok for Magic Lantern (you might need to fiddle with the firmware, but that's all).
Enjoy!
Robert
http://www.dpreview.com/articles/4701677448/how-to-inspect-a-used-canon-eos-7d
He even specifies which firmware versions you're looking for.
There in nothing specific you need to look for regarding Magic Lantern. If the 7D passes all your other tests then it will be ok for Magic Lantern (you might need to fiddle with the firmware, but that's all).
Enjoy!
Robert