Trans slipping

[AL24 <AL24@domain.elided>]

After my 16 year old sons first three months of driving my 79 Scout II The
auto trans is starting to act up. Sometimes from a stop it feels like the
trans is still in high gear instead of first. He says the trans slips into
nuetral. He also told me that the engine stalled on him one day and because
the brakes wouldnt work ( no vac) he threw it into park.(OUCH so much I havent
told him about cars yet) The shop we use hasnt looked at it yet, but said it
sounded like it might need a rebuild (99,000 miles) and said from 9 to 11
hundred $$.  Does this sound right any other ideas? I do trust the shop, they
have always been upfront with me.  Thanks for any info.