Hello ,

Been working out the kinks in my 85 535i and the exhaust is last major hurdle before it will be truly roadworthy . I've scoured the web but can't figure out the best all around option . My mechanic said the downpipe was removed . So from header back is needed . Any help would be much appreciated .

I would just take it to a decent exhaust shop in your area and have them build a single 2.5" system the rest of the way back. Have them merge the two headers after about 2 more feet of tubing, then a single inlet/outlet cat, then a resonator, then a nice sounding muffler at the back. It may be a few hundred dollars, but that's going to be way cheaper than any aftermarket option. Unless you can find a good used system near you. Shipping would be pretty bad on something that big.
The 2' of pipe before the merge is just a guess on my part. You'll want to do some research to make sure that's going to work.
Good luck

A single 2.5" from the cat back should be perfect. It is very hard to find a dual outlet muffler but if you are ok with a single outlet (you could also use a dual tip) a Magnaflow #12646 works great. I had one on my car before going to a 3" setup for my turbo and it sounds great. Magnaflow also makes a dual 2" inlet/single 2.5" outlet cat #93507 which works nicely. Justmagnaflow.com has good prices on both the muffler and cat.