All three are good options. I think you have to decide what is appropriate for your wallet, now. $1500 for a rebuilt tranny is cheap, if it's done right, and if you can afford it.
Personally, I hate to crawl under any car and wrestle anything that weighs more than me. If you are comfortable with that, go for it, you will save yourself over $500. I wouldn't install the rebuild myself because if there are problems, they will say you did something wrong installing it.
I had a 300D that developed a pretty severe leak at 160K. I too struggled with the decision. I decided to have the dealer re-seal the tranny,($900) and yes, seal the other end while it's out. I decided to re-seal it because with the right fluid level, it shifted properly. I think that's they key, if it shifts well, seal it. I drove mine another 30K and sold it to a friend. It has never leaked a drop.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)