Don't buy anything until you have it all apart, cleaned up and fully inspected all hard parts (drums, pump, gear train components, etc). Then spend a little more money so you can avoid doing the job twice or three times. If you're not sure what to specifically look for and how to test/evaluate parts for reusability, watch my
4L60E Teardown and Inspection video - I get into the weeds and cover everything I look for on these units.
Once inspection is complete, replace all of your clutches, 3-4 steels (the others are usually reusable), 2-4 band, accumulator pistons, paper/rubber, forward sprag clutch. The kit from OPT should have everything except the sprag / acc pistons so just tell them you want a Borg Warner 29 element forward sprag and the accumulator pistons added to the order.
Pump will likely need resurfacing (cover and body); you'll want to install a Sonnax boost valve and spring kit for improved line pressure characteristics. Outsource bushing removal/replacement to a trans shop if you don't have the correct bushing drivers (bearing, race and seal driver kits won't work except for maybe a couple bushings).
Watch my
200-4R 'Special Tools' video for a very cost effective method for resizing the one-piece teflon sealing rings on the input shaft after installing them. Watch my
4L60E rebuild series if you need step by step guidance on reassembly.
Let me know when you're ready to disassemble the valve body, accumulators, spacer plate and servo and I'll give you a few tips on mods you can do for less than the cost of a shift kit.