New user - 81 300TD wagon -tranny ?

[Hoss]

I just bought my second Benz...a 81 300TD with 141k miles on it. It's in great shape and I would like to go on a long trip with it but I want to make sure the tranny is in good shape. I've searched for posts on transmissions ,but I'm not finding what I need. The forward gears seem to be shifting well, however reverse gear is taking a second or two to engage. The mechanic for the previous owner (of 10 years) said it needs to be rebuilt immediately $$$$ . I understand that may be true, but I would like other opinions...Any info. would be of great help.

Is there an adjustment I could try first? or ?

Thanks,
Hoss