Okay, Let's see where do I begin. I bought my car back in 1976 with 40k miles on it. After putting over 350K on it I still drive it today. It is by far the best car I have ever driven. The only thing ever done to the engine besides tune ups and oil changes (every 3k) was a valve seal replacement at about 250K, about a $400.00 job.
These cars are in my opinion and others one of the best Mercedes ever made.
I recently bought for my wife a '00 E430, nice car but in my opinion no where near the quality and feel of a 280SE 4.5
So if I were you I wouldn't waste much time waiting, however I would recommend doing a compression test and checking a few other things (air conditioning, heat system, wipers, etc.) since all of this can add up to a lot of money if they need replacement. Good Luck! You won't be sorry if the car checks out. The best 3K you'll ever spend!!
'72 280SE 4.5
'92 190E 2.3