really need some help

i have a 1996 k2500 4x4 with a 454 7.4l motor i think it has a 4l80e tranny. the reverse went out and took it to aamco said i'm probably looking at around 2k to rebuild it. i don't have that money i have three kids my wife stays home my disabled dad lives with us along with my homeless sister and her kid. anyways i don't have that kind of money. i was looking around on craiglist and seen some of those trannys for 3=400 dollars. . does it have to be a certain year? can i use different type of tranny. i am mechanically inclined but i have never done anything like this on a vehicle. i work on refrigeration units for a living commercial and residential so i have some mechanical ability. is this something i can do myself .. putting in a used tranny. if you know what i should be looking for or if this would be over my head let me know. any info would be appriciated.
thanks

p.s. i live in the phoenix area and if your capable of helping i can put a new a/c or heat pump system in for you

Installing a transmission isn't very difficult. I would be very hesitant to install a used transmission. You don't know if/how it's been maintained or how many miles are on it. It may go out after a few hundred miles. I would check back with Aamco and see how much they want to rebuid yours if you do the R&R work yourself. I assume the $2k was for them to remove, rebuild, and reinstall it?

I would get prices. and if you do the work youll save too like mentioned.
we had one removed/rebuilt and installed for 1500 out the door at a local trans shop.