no worries mate
not too sure about how many kms before the problem occurs, sorry. but we had 150,000km on ours when we bought it and the problem was there.
in my opinion you should save up a little bit more cuz we bought and sold the car around the $8.5k mark.
if the problem does arise(touch wood), taking it to a transmission mechanic for a service won't do anything. can't remember how much they charged us, but after about a day or two the problem came back.
good luck with your car hunting