Hey Kyle, if I understand correctly, you already change the thermostat & wiscus fan?? This after your car overheated, exactly how much did it overheat? No power?
If this is the case and your engine sounds like a Isuzu diesel, then there is a major problem present, or strongly on it's way!
Sounds like this could be piston knocking, or maybe bearing knocks. ( Maybe sticky valves if hydraulic activated )
Thats my opinion, I'm sure some of the other forum experts will know better, I'm just playing it by ear in a 'general technical sense' .
Posting a thread, have a look just below the white google search bar, there is a few headings, select the BMW fanatics forum, and then the next applicable one etc, eventually right hand top corner you can select 'New Topic' , and off you go!
I also got it with assistance of friendly guys on the forum!
http://www.bmwfanatics.co.za/forumdisplay.php?fid=14
Good luck man!
:=):