Here's the guy you need to get in touch with:
http://www.914world.com/bbs2/index.php?showtopic=56892
Dr. Evil will do a top notch job on your gearstack overhaul and work with you on costs with his huge stash of good used parts.
Unfortunately, the 914 trans can be picked up cheap (which seems like a great deal at first), generally around $200 or less - but that can only be considered a core trans. The OH costs can range as little as $500, or as much a $2500 (and more!) depending on what needs replacing, any modifications, HD parts and/or used parts you can use. Lots of guys get 2 core transmissions to have more spare parts available to build one good one. The bearings and 1st gear parts are the killer, plus you have to start out with a core with good R&P gears (no pitting).
The above link will walk you through checking the trans and pulling the gearstack for OH.
Jeff