Some good advice, however a few additional thoughts:

re I would be careful about buying "used" from Colombians. I think your plan of buying the bike in South America is smart. It would make a lot of sense to buy another foreigner's bike down here as long as you can get it for $1500-2000.

- I’d be careful about buying a bike from foreigners as well..

re I wouldn't limit yourself to starting out in Bogota. If there is a good deal on a used motorcycle in Buenos Aires, start in Buenos Aires instead.

- Makes sense.

re Not sure how much sense it makes to go to Venezuela and then Brazil by way of Manaus. I have wanted to go to Venezuela for almost 20 years now, but it has always been just a little too dangerous on my risk meter. Did you know that it is more expensive in Venezuela than Colombia?

- It has improved, however it’s interesting that whereas a few years ago fuel was being smuggled from Venezuele to Colombia now the smuggling is in the other direction! The journey via Santa Elena de Uairén, Bõa Vista, and Manaus is well worth it in my view.

re If you start in Argentina, take a tent and cooking gear with you. If you start in Colombia, don't bring a tent or cooking gear because rooms and food are cheap and there are snakes and spiders.

- Take into account costs everywhere, Colombia, Ecuador, Perú, Bolivia, and Paraguay are inexpensive, Chile and Brazil are expensive. Argentina is a financial mess however you can find economical rooms in Argentina. Snakes and spiders are not particularly common in Colombia and equally can be found in Argentina.

re Do not ride at night in South America.

- Agreed – or at least, if you do then do a quick risk assessment – sometimes it makes more send to continue half an hour carefully than camp at the side of a main highway.

re Be careful with blind corners in the mountains.

- Agreed - basic safety, always ride so that you can stop within the distance you can see to be clear.

re My advice for Patagonia is just ride the bus down there, or fly down there. The Southern half of Argentina sucks. The last 300 kms South is Patagonia, which is very beautiful. Basically everyone just rides the Southern half of Argentina because they want to tell their friends that they rode the whole Panamerican Highway.

- Hmm – we all enjoy different things, however in the Southern half of Argentina (prior to the southernmost 300 km) are: Peninsula Valdes, Bariloche, Lago Viedma, two different petrified forests, access to Torres del Paine, and half of Ruta 40. Lots to enjoy, in my opinion, other than Perito Moreno/El Calafate. Ushuaia itself is not particularly interesting – a duty-free shopping centre and one good restaurant.