I've got both a '79 450SL and an '82 380SL. The '82 380SL has a 560SL engine and transmission installed. I like them both for different reasons. The '79 is in great shape after installing a new interior (except for the door panels) and installing a digital servo for the ACII climate control system. It has lower? gearing than the '82, so the engine spins faster at any given RPM than the '82 does. It is a very nice handling car and rides well. It doesn't get as good mileage as the '82, however - around 18 mpg on the freeway.
The '82 is my daily driver - it's got LOADS of power, gets almost 22mpg on the freeway, and handles well. I much prefer the gearing on the '82 for higher speeds, as the engine turns about 1K lower rpm than the '79 at 75 mph. The '79 is definitely up to handling the higher rpms, though, as I bought the car in Florida and drove it to the State of Washington, over 3K miles, at speeds of up to 85 or so in three days. It just drinks a lot more gas than the '82...
There are two main concerns on the '79 - the front subframe will fail if it hasn't already been fixed or replaced, and the ACII climate control system will fail if it hasn't already. New vacuum pods and a digital servo update will fix the ACII system, and a new subframe will fix the front end trouble. (Still serviced under a recall, I believe.) A new, improved subframe is available from Performance Products for around $650, and the digital servo is available from them for around $699. Both items are also available from this forum's sponsor, as are the vacuum pods.
If you can afford it, I would hold out for an '87 - 89 560SL. They have a much improved front end, and there are many, many refinements throughout the car. They cost more, but I think they are worth it, and you can generally find one with lower mileage and much less or even no rust. Just my .02 worth.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C