Think of it this way. What else are you going to get for $150.00? As for the tranny rebuild, read this thread, it may just give you the insperation!!!
http://www.peachparts.com/shopforum/showthread.php?threadid=27603&highlight=ultimate+DIY
But, as previously mentioned, before jumping in in rebuilding the trans, check it out, it may be a simple fix. (though with 300,000 miles?) Has the tranny ever been rebuilt before?
Again, i would go for it! And if you fail, well, get another 190e and between the two of htem get one good one!
Iggy