1997 M3 auto - Transmission woes

I recently picked up a 1997 M3 4dr auto and have been working through a number of issues. I don't believe the previous owner did any maintenance - I won't go into the long list of items I've already addressed but the car is coming back to life with a little of my elbow grease. The biggest issue that I need to now work through is a transmission shifting issue. Basically the car will not shift into 3rd or 4th. And throws codes P0734 and P0733.

I did a transmission drain drain yesterday. Dropped the pan, replaced the filter, filled up with fluid. No change - still won't shift into third and throws the codes.

After watching a few youtube videos I see guys inspecting the innards of the old/dirty filter, I tore into my old one. I found quite a bit of metal and possibly black plastic shavings. I now suspect I'm in for a rebuild/replacement transmission. Thoughts?

Anyone have any good hints on places that rebuild / remanuf the ZF 5hp18 (A5S 310Z)?

Thanks,
David

Why rebuild this transmission when you can find good used ones with around 80K miles for not a lot of money?
Check the guys doing swaps for 5 spds and if the tranny is good and they stand behind it change the fluid out with Esso LT 71141 and it should take you another 100K to 120K miles. Mine went over 200K with no fluid changes.
Also I recently picked up a nice used ZF auto for out M3's for 103 bucks. It is not sitting in my garage has all of 80K miles so they are out there.
There is a ATSG manual for 50 bucks for our 5hp18 with exploded view and all part numbers listed.

If you choose to rebuild I would get this manual and than see if a good automatic rebuilder would take this on as a project. They have a couple of special tools needed to do the rebuild that home wrenches don't and that makes all the difference. However the manual is essential if you try to do this yourself.

So it looks like I'm faced with a transmission replacement. The one in the car is toast. So I need to choose:
1) used (no warranty) - probably the least expensive choice
2) rebuild (with warranty) - much more expensive but possibly worth the peace of mind

Anyone have any good sources for either? Pros and Cons?

Update: I have removed the transmission and have spent several hours researching replacement options. I'll probably buy a used one instead of reman as they are generally less than half the price. The bolts especially around the top of the transmission are pretty darn tough to remove - good to have a helper. But yes, it's out of the car. Plenty of other parts to replace at the same time (exhaust hangers, flex disc/guibo, and a few others).

My question is: Assuming I buy a used transmission (automatic), can anyone give me a good idea of how to remove the old fluid in order to replace it with fresh/new fluid? Dropping the pan/filter only allows you to replace a fraction of what is in the entire transmission. Thoughts?

If you dump the Esso out of the pan and the torque converter you will get most likely 6 L and it holds what 7L's?
That is my suggestion which will get you most of the way there.
One point. The pan and install must be SPOTLESS. If you get any lint it can end up in the check balls and cause a poorly operating tranny.
Good luck and yes BMW uses a lot of fasteners and not in the easiest of places to get at.