The most important thing to check is the timing chain: the  original
single chain was almost certainly converted to a double-chain by now,
but the single chain, the upper guides, and the chain tensioner should
have been changed roughly every 100K miles. If they haven't, and the
chain is streched and/or the guides worn, either could break, and
you'll end up with a 5-6k bill. (I believe MB recommends that the
lower guides be changed at around 200k miles, which will cost you
around a grand, but I understand that the lower guides hardly ever
break).

Also check the engine for oil leaks, as the head gasket, particularly
in the left rear corner on the driver's side -- tneds to develop
leaks. However, if you don't mind adding a quart or two every, say,
500-1000 miles, you can live with the leak.

Another thing you should check to see if it was ever replaced is the
A/C servo -- a large black plastic cannister with a lot of colored,
plastic vaccum lines coming out ot the top -- since these usually
crack or malfunction, and a new one costs over a grand. But if you;re
familiar with the 450sl, you probably already know about this.

The iron head 450 engine is reputed to be more powerful and durable
than the aluminum head 380, so don't expect a lot of pick-up from the
latter.

Good luck, if you decide to buy it. I have a 380sl with 100k miles on
that I love, but it always needs something, and if you can't do the
work yourself, be prepared to pay for it.

Be sure you can live with its 155 HP engine before buying.