I'd look for one a few years older personally. As already mentioned, the climate controll servo unit introduced in '77 is a terrible design. I'm not sure I agree with the post about the 380 having more power though (The 380 has 155HP and a 1980 450SL has 160), but neither are powerful by any stretch of the word. Also, the 380, unless previously converted, has major timing chain problems. Take a look at this article for some recommendations and an indepth look at the changes made over the years:
http://fly.hiwaay.net/~gbf/107/rt.html

I owned a 1980 450SL for about 4 years myself during which time I spent nearly twice what I paid for it on restoration (had a great time doing it though). I mention that only because I've noticed (here and elsewhere) that some seem to feel that acquiring these cars for more than a few thousand is folley. Granted, they're older cars, but I wouldn't have sold my 450 for anything less than it was worth and most people selling anything more than a project car wouldn't either. The least expensive SL is always the one that costs the most.

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-- Aaron

'87 MB 560SL - 45K
'80 MB 450SL - 180K (Sold )
The rest: '83 BMW 533i - 286K, '04 Ford F150 - 7K, '99 Yamaha YZF-R1 - 10K