Several thoughts:
As to changing the RD on the R3, given the fact that my medium cage 105 RD should work with as high as a 32T cog, I'm inclined to use the 105 RD
I'm not sure that this assumption is correct. I could not get my long cage 105 (triple) RD to work to my satisfaction with a 30 tooth cassette, and I've swapped a lot of components. Also swapping cranks, especially from road to mountain, can involve chainline issues in some cases.
nun
I've toured on a cervelo RS and the only change I made to the gearing was swapping the 50t ring for a 46t. If you want to go the whole hog I'd use a mountain double, forget the triple, and pair it with a 11/36 cassette and new rear derailleur
If I read nun's past posts correctly, he goes ultra light, and modified CC tours. I know the beating our touring bikes have taken, and I'd be a little leery of using a carbon frame bike. Not that it couldn't work, I'd just feel it has to be babied too much. I would not consider even light touring on my "good" road bike, and it has an aluminum frame.
hueyhoolihan
considering what you already have and that you will be pulling a trailer, i'd dump the 50T chainring (completely useless IME) and go with one chainring and the cassette you have. that gives you a range from 25 to 80 something gearinches. plenty low and more than high enough.
I agree that the 50 T chainring is useless when pulling an 80 lb. trailer. However, I disagree that the 25 inch low gear is low enough. I run a 44/32/22 with an 11/34 which gives me a 17 inch low. I don't tour with a trailer but I use a Bike Friday trailer a lot to haul groceries and other items. With 60 lbs. of groceries I am really glad for the lower granny gear when I'm pulling that trailer up a short steep hill on the way home. I prefer panniers, but my loads seldom exceed 35 lbs. I've also used that granny gear often with my normal loads. However, I don't generate much wattage either
"other items"
I put together detailed numbers and if I were to get a touring bike (would be a Disc Trucker), it would add 3K $ w. taxes to my pre-trip costs since I don't have racks & panniers and don't want to go the second hand route
For a one time shot, you may get away with a regular LHT. I run a good set of cantilever brakes on mine and it stops just fine. A stock LHT is about $1200 (U.S.), and can be sold after the trip for 75-80% of the original price.
A Surly LHT or Cross Check (or any number of other bikes) would haul your trailer very nicely.
I've done lightly loaded, 20-25 lbs, tours on my "not so good" road bike, and it works just fine. But it seems like you are trying to rig a Mazda Miata to pull an 18 foot travel trailer.
P.S. Something I didn't think about until a few minutes ago on my way back from the post office with some freshly pumped up 28 mm tires @ 100 psi rattling my fillings. The R3 may not be able to take a tire size larger than a 25 mm. I've toured a few times, nothing longer than 3 weeks, on 25 mm tires, and again it is doable. However, the comfort of a little larger tire on a long tour is something I appreciate. I have used 28 mm tires a lot, but have recently tried 32 mm tires for an extended trip. I'm still vacillating between the 28 and 32's. A good compromise seems to be the light weight Conti 32 mm Ultra Gatorskins which definitely won't fit your R3.