YES. the tranny is slipping.

I'm not following the logic here.

If the B2 piston is the problem, the mechanic should be able to fix that for less than $200.

If he's going to remove and replace the entire transmission with a unit that you found from a forum member..........it's going to cost at least $500.

I presume that you're not about to R & R the transmission yourself??

The least costly approach is to troubleshoot the transmission that you currently have in the vehicle. Hopefully, it's repairable at a reasonable cost.