Hey Dwagenheim, I'm actually on a break in Eugene right now, lol.
Anyways I recommend you get the trailer, I've met several tourists now who pulla BOB trailer and they all said they love them. One French guy told me he uses his off road all the time and it handles well. You and I weigh almost the same but I have been doing the 4 pannier thing and destroyed one rim already supposedly after running over a curb straight on (less than 5 MPH). If you think about it, that's really not much of a shock and I was running a 40 hole rear wheel on Velocity Dyad rims with 2mm straight guage spokes and a 4 cross build (built by a 260lb tourer) so if I can blow that wheel up on the streets you'll definitely be able to do it on the rough roads and trails of South America.
As far as the waterproofness of the Ortliebs they have been great so far. Rode through numerous flash floods in France and several all day sprinkles though no 6 hour tropical storms like someone else on here. My front panniers are just cheap MEC jobs and they kept my stuff mostly dry too though. I think it's wise to get a set of Ortliebs and a set of water permeable panniers too if you were to go the pannier/rack route but I don't recommend it. The only downside I've heard about BOB trailers is that they can fishtail on you above 25 mph but I gather that can be overcome with good loading and how offten do you go that fast on a touring bike anyways?