sorry!

guess i posted my question in the wrong place.
i am new to the forum.
my friend gave me his 83 320i. it runs well but needs a bit of work.
i do not know very much about these cars but would like to learn, as i would like to fix this car and through time restore it. it does not have a muffler or the ahngers and hardware for one. everything is missing from the flange back. (the flange after the middle section).
purchasing one locally would cost $108.00 for the muffler not counting hardware. I could buy one online for $60.00 and 25.00 for the hanger package. Do i have to get a muffler that is specifically for a bmw? could i get flexible tubing, a high performance muffler, and install my self in any fashion that works? or....what are my options.
thank you.