Well, a couple of things to look out for but I think none important enough to not buy the bike. But it gives you a better position barganing if you know a few things:
For all 550, ORIGINAL parts that are hard to find and/or expensive:
-Exhaust, especially the 4into4's can cost more than a whole bike in good condition. They always look good from the outside, but are rusted from the inside, have a good look at the bottom and don't be afraid to tap it with something to check thickness and rust. 4into1 are usually less of a problem, and there are aftermarket options available at a price.
-Sidecovers. Uncracked sidecovers in the original colour are best.
-Most large crome parts like fenders etc are expensive to replace. Get one with good chrome, unless you don't care about looks.
-An undented tank in original colour is great, also check it comes with the key for the latch and check the inside for rust (can be fixed).
The 77&78 have different carbs, and a hard to find airbox. So don't buy this without the original airbox. In fact, don't buy any bike without the original airbox, most that are modified to pods are not rejetted properly.
With the first years 550, there was occasionally a problem with the rocker arm shafts rotating and wearing in the camcover. Later 550's were changed to prefent this It can only be checked if you take the breather cover on top of the engine off, and most people don't do that when buying a bike. On a low(ish) mileage bike it probably won't be an issue. Worst case scenario if you don't check it, replace the camcover.
http://forums.sohc4.net/index.php?topic=20408.0Leaking headgaskets are rare, if the bike leaks oil it will likely be an o-ring.
Rust is a problem with all older bikes, 550 don't have big trouble area's exept the exhausts. Also check the fork tubes under the headlight mounting brackets. Pull back the plastic boot under the lower triple clamp, if there is rust up near the lower triple clamp, then the tube is probably rusty the whole way up. A little rust is ok, but a lot will make for a weaker tube..
Testdrive as many bikes as you can. I'm sure if you ride a good 550, you won't be tempted to buy the 750 so soon. A 550 should pull strong from 3000rpm in every gear without obvious dips. Don't be scared to rev it to redline, they like that. It should also start up easily without warming it first (feel the engine when you arrive).
Don't buy one just because the colour is better, look at the total state of the bike and the parts that are missing or need replacing. This will safe you some money later on.
Good luck, and pictures when you buy the bike!
