Look at www.collectorcartraderonline.com.
It won't tell you which engien is best but it certainly will give you a selection.
Watch out for the 380sl. They have a weak timing chain that when it breaks can cost you BIG bucks. Make sure, if you look at a 380, that it has had the double timing chain conversion.
If you like the sl style you might look at the slc. They are 2/3s the price, a much safer car and by far better looking. I have had two of them and they are great care.
Both the 280 and the 450 engines are good, but, as with all MBs you better get your checkbook ready for some shock treatment. The part you will pay $50 for on a Chevy or Ford you will pay $300 for aon a MB. Also there are more mechanics that can repair Fords and Chevys than MBs and therefore the labor rate and overall cost can be less.