2000 Hyundai Elantra Please suggest

When I put my car in the drive mode, it takes some time to get the acceleration and then there is pull/jump when it tries to ship gears internally and I am guessing that it is taking a while before shifting from the first gear (internally as it is automatic). I took it to AAMCO repair center and they did some electrical analysis and told me that they have to disassemble the transmission and fix the seal problem. I was told that this fix would cost me ~$900 and replacement of transmission would cost $1500+. I was also told that the transmission fluid was dark in color. Also the gears are in good shape and they told me something about flares. Please suggest on what needs to be done as investing that much on this car is not worth it as I don't know how long it is going to last or if I choose to sell it off, I will not be even be getting to what this repair costs. If I choose to get the transmission fluid changed, would that help? Also how long do you think that this car is going to last with the current issue.

Thanks in advance for your time.