HI Chad
hoping to put a few positive words your way...
as far as travelling two up without bike experience I did it on a bike that was way too big and heavy for me and with just two months experience on a journey that took in Asia, Australia AND Africa, so it can be done and we had a lot of fun.
I travel pretty cheaply and for me the biggest expense is petrol unless in Venezuela where it was 5 cents a litre. so two up makes sense for a cheaper ride.
It can be cheaper than public transport depending on bus prices etc, after years of doing backpack and bike travel, I can't see myself going back to buses. What can make it cheaper is carrying a tent as camping options are easier to find on a bike.
Look for posts by Nathan who travelled for a couple of months two-up on a 350cc bike through the Stans to Mongolia last year - proving it's possible.
You do not need to do much off-roading in South America if you don't want to, I travelled around for 10 months and could have done almost all of it on tarmac..if I had wanted to
as for buying a bike out there - I have to confess that's an area I have little experience of as my bike has lasted for 13 years (and counting) of great travelling.
keep an eye on the For Sale notices here on the HUBB, quite a few are in Buenos Aires, so if you have a choice of destinations maybe go there- I'm lucky in that I only ever had one doomsayer before I started bike travel, it seems like you have a few more.
You will have a fantastic experience, be sure to share the tales of your travels with us.