There's no way the transmission will be shot on a '98 740 with that many miles. I suspect you have a later tranny that should have been recalled for transmission ring-gear wear - this problem affected late 98 and 99 cars.
Check to see if the previous owners had recall 99V005 carried out and if not, you can just take it to your BMW dealer who will fix it for nothing.
Hopefully that's your problem, otherwise it's going to be horrifically expensive - the best place to get a new tranny would be from a written-off model at a breaker's yard; if you get BMW to do it, you're probably looking at paying half what you paid for the car
