Should I fix my 2000 Hyundai Elantra (126k) transmission or just junk it?

I know very little about car repair - maybe because I've been very lucky with cars, but not anymore. My car broke down a few days ago and the shop said the transmission needs to be replaced -to be more accurate "Ran computer scan and found codes for incorrect shift ratios for #3 & #4 gears. has code for controlled computer output circuit problem. trans fluid is dirty/smells burned.needs transmission --PO733,PO734,P1634 (dont know what these mean)" for $2000-2500 approx. I bought the car 03/02 used for 5500$. I have had no major problems since then & do like this car. The kbb is $3060 private party/trade in $1760. It does have some minor body damage, also needs AC belt replaced, battery and valve cover gasket (?).My brother called for salvage trans. and found 1 for $700 but he wouldnt have time to install it. Could anyone with some car experience let me know what you think -I'm leaning towards getting rid of it. Should i take it in for some more mech. opinions?It barely runs now.Thanks!