Get the Tamron if you need a shallower DOF, or need faster shutter speed for lowlightings, or on a tighter budget. Get the Canon if you are looking for built, slightly better image quality, faster focusing and the L branding.
Not necessary, depending on how the previous owner uses it. If the overall condition's gd then it's definitely better. Else i would sugest u save more and get a new 17-40, now around $1100 only.
Just to state an example, one of my friend bought a 2nd-hand older 17-35 f/2.8 and its zoom ring is very loose. That means it's heavily used but it doesn't matter to him. So the bottomline is, a used original lens doesn't mean is still better than a 3rd-party one of similar class.
I would go for the 2nd hand Canon L 17-40 because of it's track record, build quality, greater re-sale value and the bottom line factor:top-notch repair and after-sales service for it unlike ehem, some brands where all you get is some crap excuse and everything also have to 'send back to Japan, have to send back to Germany bla bla bla'.
If you need more reach, flip freak suggested a Sigma 18-200 but the wide end suffers a bit, something that may bug you to no end for travel applications like landscapes and especially architecture.
Actually, coming to think about it, 17mm is also no great shakes - it equates a FOV of a 28mm lens on a full frame when used on your 30D, so it a little wide, but not that wide. Personally, I would find 17mm too restrictive, but that's just me.
Or you could get adventerous and go with a Sigma 10-20mm and a cheap 50mm prime (or a top-notch, super affordable, premium quality manual focus 50mm prime). Not a bad idea as it might force you to see things a lot more creatively.
A lot of these things depend on your personal vision and ways of approaching a given subject.