I can understand the need for a valve job at your mileage, but although not impossible, I would be extremely surprised if you would need a complete engine rebuilt.
The bottom end of those engines are extremely robust and very rarely need work. There is a taxi operator in our town with a 1989 300E (same engine) with over 650K on the original engine. Except for valve work, the bottom end of the engine has never been touched.
On my car, the valve guides/seals were done at about 125K and the valve seals were done again at 240K. The top of the pistons and cylinder walls look as if they were still brand new at 125K.
$1,300 to $1,500. for a complete valve job seems OK.
jackd