124 transmission life???

[new guy]

I have an '87 300E w/ 150,000 miles on it. Within the last 2 weeks, an ATF leak has developed on the passenger side of the transmission. I have asked my mechanic what is required to fix it, and he told me that most likely the transmission would need to be dropped in order to replace the seal that has gone bad. (I don't know what seal has gone bad, but I know it is not the pan seal) He told me that dropping the transmission can be a risky deal being that it has 150,000 miles on it, and I might just want to rebuild it at this time. How many miles should I expect to get out of a transmission??? With the exception of the leak, it has been worry free, and still performs well.