Torque converter question

With a 125K on the converter, a lot is going to depend on how much sludge is in the pan. If the pan is nice and clean then go ahead and flush the converter out. Remember there is a one way roller clutch in the converter and if it's questionable, flushing won't cure any problems in that area. It's difficult to flush a converter out. There are all kinds of areas that the material can and will get impacted in, and flushing it with solvent won't move it. What will help clean out the crud in the converter is the nice 180 degree fresh transmission oil you'll have in your rebuilt transmission. ATF is one of the best cleaners there is.
Also, check the condition of the converter hub. The clearance between the hub and the pump bushing is about the same as a set of rod bearings. If the hub isn't mirror smooth, you will in all likely hood end up with a leak.