Ve sv6 sport wagon 5L40-E Stuffed need replacement

I’ve not long purchased a sv6 sport wagon for the missus as a daily/ grocery getter. all seemed good during the Test drive and on the drive home both only short trips <20mins, missus drove it for a week and didn’t mention anything. Following weekend We jump in and go for a decent drive into town after about 20mins to half hour of driving (once the trans warms up) I notice it’s Flaring changing from 4th to 5th and slipping a little in 4th , me with high hopes decided to do a service (filter, oil, bottle of Lucas stop slip and gasket) Hoping it’s just old dirty fluid... nope, actually made the problem worse. Oh and the fluid was filthy. Took it to a couple of trans specialists this morning and both say it needs a overhaul quoting $3500-$4500 only paid $3800 for the car.
Still in talks with the bloke I bought the car off over the situation he reckons he knew nothing about it But is being reasonable.

if I buy a used second hand 5l40-e box and fit it myself is there anything I need to look out for Like certain year models / series codes ect or will any 5l40-e box work? Cheers