Err, depends what's wrong. A fresh torque converter and oil seal should be painless enough but to delve into a worn box is not a good idea for someone who has no experience. If there is an internal fault I would be looking for another box. You need special tools to set the clutches, bands and pressures up, together with the experience to know what needs replacing, or doesn't.
The seals fail because the oil gets very hot which hardens the rubber seals and 'O' rings. Do you have an oil cooler? If not you should have. If you don't that could be the root cause.
My Marlin had two BW 65 boxes with it, I didn't trust them (not used since 1991) and no oil cooler. I went with a 5 speed manual for reliability. The only real issue with that is you need a clutch pedal.
I am not anti auto, I have used and enjoyed them for many years but they are very expensive to fix if you can't do them yourself.
PS. If you do pull the box, undo the torque converter from the engine first, (usually four bolts screwed from the front of the drive plate into the torque converter) and remove the torque converter WITH the gearbox, don't be tempted to pull the gearbox out of the torque converter in the car, that is a very good way of wrecking the front gearbox seal. Once the gearbox is out, then GENTLY pull the torque converter STRAIGHT out off the shaft, don't let it damage the seals and shafts by letting it drop as it comes off the shaft. Re fit equally carefully.
Last edited by 8 Valve Ed; 6th March 2015 at 22:15..
Reason: Adding PS.