The 9 speed Tiagra STI is good enough, and works well with cantilevers.
STI, and any indexed system, can be a bit fiddly to set up. You could reasonably expect a 10 speed setup to be 11% more fiddly than 9 speed. Note, too, that you typically don't get any greater range with 10 or 11 speeds than we did back in the dark ages with 7 speeds. You just split the total gearing range into more slices.
One thing that's hard to deny, though, is that chain and cassette prices are significantly higher for 10 than 9 speeds, and there's another jump between 10 and 11. It's not a major expense compared to the overall cost of a tour; and it may not matter to Pete, who gets freakishly long lives out of his chains; but your credit card will feel the difference when you order three extra chains and a new cassette.