I'm looking to get n+1, I want another bike with a triple, and something a bit carbon-ier and a way to keep gravel tires on my current bike, along with rack and fenders etc for sloppy use.

There's a 2007 Roubaix up for sale locally, wondering whether there are any reasons not to jump on it, and/or what a good price would be... I can fiddle with, tune and replace the odd bit here and there, at the moment the price is probably below what I was budgeting just to buy either the frame, or the parts list for a triple build I was hoping to do this coming spring.

Are frames of that vintage a suicidally bad idea, are they much worse than the current crop, or is it worth considering? Bear in mind: I'm cheap, and I wouldn't drop the money on a bike with Ultegra and so on new, so this is a way to get what I was actually looking for with a little bit saved alongside.

The bicycle blue book website seems to be pretty good on fair valuation for used bikes: Bicycle Blue Book - Used Bikes

I would think an old bike is only as good as how well the owner took care of it. If it was owned by "Crash" Magee, amateur racer, then no I wouldn't buy it. If the owner took care of it well and never laid it down, then I'd have no problem riding a 2007 carbon bike. The other caveat is it also depends on how many miles have been accumulated on the parts, as if everything is old and worn, then you'll have to spend money now or soon on stuff like the chain, cassette, tires, cables, cable housing, etc.

I'm okay in principle with the idea of budgeting the money for a full tune-up if required (either by my own hand or someone professional, depending), including a new chain goes without saying, and a cassette would be discretionary. I'm also figuring on budget for stem, seatpost, saddle, possibly bars depending on fit/comfort. Pedals would be additional as well, obviously. I'd like to know that I can get a look at it in person and see if it's got any signs that are worrisome... but, that aside, any reason to avoid 6603 instead of 6703 Ultegra? I've heard that the non-hidden cables actually had some upside in terms of performance and smoothness, I've got them now on 9-speed and don't mind them much, not sure what other changes there might have been between the two series.

It looks like original tires, without much wear, from the photos... which would imply that it's been lightly ridden, I would hope/imagine. The rest seems remarkably clean, though not in the way of something that's been detailed, just unsullied, but there are some clearcoat chips visible.