It depends on how many transmissions as a "shade-tree" mechanic you've rebuilt. Do you know how to evaluate the internals, get the correct parts, and make the reassembly so you won't have to pull the box and correct your errors?
There's a myriad of tech information when you search "T5 rebuilds" and I'd suggest that before you jump into the fire you take time to insure you won't be wasting your money and more importantly, your time.
At an average of $150-200 for a salvage yard T5 I'd have to be really hard pressed to undertake a rebuild as an OJT project. There will probably be those that encourage you to jump right in and have-at-it and hopefully, they'll be there to bail you out when things get dicey.
Dave
