here's some quick maths:
518: $600 (CL/offer up)
comprehensive rebuild kit w/ shift improver & upgrades: 800
torque converter: 400~1200 (so let's split the baby and call it 800)
mount: 400
lock up/OD control kit: 150
yoke if you didn't get one: 50~100
shortening drive shaft: 150
trans cooler kit: 100
misc odds and ends such as fluid, biscuit mount, dipstick, wiring supplies, etx: 200
so you're in 3200~3500 easy money
this surmises that A: all the hard parts in the trans are good and you're not upgrading planetary gears, and B: you're doing all the work yourself, as well as C: you're okay with cutting up the car.
the OD option has all the pros of not spinning the motor to the moon at highway speed and getting *slightly* better fuel mileage which would only be important if you were long hauling it, tbh. but suffers all the cons of spending a grip of dough, a bunch of time and cutting up the car.
vs spending $500 for some gears and saying: ehhh, that's good enough.