This box is not a easy DIY at all, but not that expensive as auto's go.
The secret to long life with these boxes is OIL COOLER, take off the output pipe from the gearbox(highest one) and put between it and the main rad a additional oil cooler.
The ones I use are for WINNEBAGO MOTORHOMES, about 3/4's the size of the main rad, they do work well, always had auto's and always put on these coolers.
My old sierra auto(A4LD) was modded with a shift kit, and used to cook my left foot, seriously it was very painful on a long journey, after fitment of the cooler the box was just warm instead of scalding and impossible to touch.
You can get shift kits for the mondy boxes, so they trick the ecu into a far more sporty mode, around £60-£80.
Just use a cooler, change oil every year with fully synthetic, and it will outlast the car.
My last sierra did over 230,000 miles on a rebuilt A4LD with cooler, at 180bhp, and was driven hard.
tabetha