Speaking of which, what are the best things to look out for other than the obvious things like fork seals, notchy headstems bearings, etc? How often should timing belts be changed? What would be red flags with these beasts when looking at a potential purchase?
I had an ST11 and loved its smoothness. There's plenty of power for our purposes and the range is all you need- they used to be popular in the Ironbutt Rallies where range is vital.
The timing belt must be inspected at 120K and replaced at 150,000. Every inspection I've heard of resulted in it not needing replacement, and the belts replaced at 150K were still in good condition. It's just that a belt failure is catastrophic for the engine that they get nervous about them. I'm pretty sure they are a quieter engine than the 1300 which has a chain drive. So when buying, you must have a service history for the engine, and if it's not forthcoming you can haggle vigorously, because the seller hasn't got a leg to stand on.
And really, that's it! Sure, all the usual issues like fork seals, pedal rubbers, seat cover, pannier appearance, locks- where do you stop with an old bike? But as you say, if you're prepared to lavish some TLC on her, the engine will be the least of your worries.
There's hours of reading in the ST1100 section of this Forum, and another huge amount on the USA Forum, so you can easily become an expert!