The Topeak beam rack are good but we did have problems. My girlfriends rack went on the il Primitivo last year and then was used all through winter in London. Just before we were due to leave, I had to take the rack off of the bike as she was using her mountainbike but the bolt was rusted. I used WD40 and twisted the bolt back and forth and got it off, however, on day 4, at the end of the day, the bolt broke. I believe that I stressed the bolt trying to undo it. We had a bolt with the same thread so we were lucky, but on day 11, this one gave way as well. Luckily, my girlfriend can speak a fair bit of Spanish so we found an iron workers place and he sorted it out. Looking at her rack, it had done around 4000Ks
Once back on the road, we found that the rack was moving around and when we looked at it, the rubbers were wearing out. This happened on all three bikes.
Having read what you are going to do, unless you are off road, you should have no problems
As far as water goes, that should not be a problem either, but on the VDLP, you need loads of water, even on a bike, as the villages are very scarce. We did have 6 kilos in the bag, the sleeping bag was in the top box, and I we had our clothes in the side in waterproof bags. Personally, I would not go anywhere without a camelback as it kept also a spare tube and pump inside