I for sure wouldn't buy a used transmission with 100K+ miles on it, you're just begging for it to go bad in a year or two also. I was quoted around $3400 installed for a new Jasper transmission or $2200 for a rebuild of my existing transmission. For that price (or less if you do the work) you could do a 5-speed swap and get a transmission that isn't even supposed to be serviced until 150K miles. Good reading for you:

VT Maverick's 5-Speed Swap