Welcome and good luck on your search. As you well know, all bikes have their quirks and its no different with the Rocket. Early models tend to have more issues than later models.
I have an '06 Standard in the faster color Black. The reds look nice but you must remember that you sacrifice something in the name of vanity.
If you are looking at an '07 or earlier, especially anything less than 15k miles, you will most likely need to replace the Primary Throttle Position Sensor. The earlier parts will have a black interior whereas the replacement part will have a blue interior. The problem manifests itself as the "high idle" issue where the throttle seems to stick around 2000 rpm (+/- 300 rpm). Since the TPS is basically a rheostat, it develops a flat spot around the position held for normal cruise. Most Captains begin to experience issues somewhere north of 12k miles.
'06 and earlier (and the rare '07) may also experience the "paint can rattle" issue. The early model cam chain tensioner had some issues on but it wasn't widespread as the TPS issue above. The "paint can rattle" would occur after a cold start. Under easy acceleration you could heard a knocking sound as you passed through about 1500-2000 rpm. The noise would go away after the engine warmed. Just something to watch out for.
Lastly, the only truly mandatory mod (and I'm sure others will agree), is to add a headlight relay kit to any model that you end up with. In Triumph's infinite wisdom, the headlight circuit passes it's full power through the ignition switch which has lead to numerous cases of ignition switch failure. This is a must do and to ignore it is risking being left stranded somewhere. You can roll your own or there is a high quality kit available from a Canadian living in Japan. He has a kit specifically made for the Rocket which makes for a truly easy install. It can be found here:
www.easternbeaver.com
I have 40k on my '06 and would not hesitate for a second to take off on a cross country trip on it. Some have reached 100k miles with several upwards of 70k.
Again, good luck on your search.