Could be the pump, could be seals too if it doesn't have the problem once warmed up. The first and last time I rebuilt a trans was 28 years ago, C4 if I recall correctly. Yep, it worked, of course the trans instructor was there if I had questions.

Something about the small and the way trans fluid feels I never cared to work on them after that.
That $400 might be money well spent though to have it built by a pro (preferably one that does hi-po work). If you mess up and it doesn't work or if it doesn't last long, your out the money for the rebuild kit, possibly the converter and a tow.
Also their "seasoned eye" might spot a small problem that could cause major problems down the line.