Hi Mike
Owned mine (2000 DB7 Vantage Volante manual transmission) for three years now and have suffered no real issues.
Mine was a low mileage car also (24,000 miles) and had already had its coil packs and fuel pumps changed by its previous owner (common problem)
Other areas are radiator and header tank leaks (check it has a aluminium header tank instead of the plastic one), the rad's seem to leak where the aluminium core is crimped to the plastic header tanks.
Power steering reservoirs also can leak, (mine is on its second tank which is leaking again), a by product of this is that the fluid can get on the r/h metalastic subframe V mount and cause it to fail (changed mine, its a jaguar part so is cheap)
Clonks going over bumps is usually the upper shock mounts (also a cheap Jaguar part sourced fix)
Mine has actually been more economic to own than I originally thought it would be, so I have been spending some of the money saved on doing fun upgrades to it.
If you like your XKR, you will definitely like the DB7 Vantage, I have a few Jag's as well and find the DB7 has a similar feel to it, also many of the parts used on the DB7 were Jaguar sourced so many parts can be sourced much cheaper than via Aston Martin.
Being in Texas you may want to test the A/C to make sure its blowing cold, as some DB7 vantage owners complain about poor A/C efficiency (mine works well and is ice cold in the summer).
Another issue seems to be a ticking noise that seems to come from the valve train at idle, mine doesn't do it , but some cars have the tick and seem to drive their owners crazy trying to solve the problem.
Could be its just down to lack of regular oil changes, have read even the cars with the noisy lifters run fine still.
Another slight issue is that they seem to set slow 02 sensor codes frequently, mine will If I don't drive it hard enough,a reset via a code reader usually sees the check engine light go out for a couple of months.
So I think they are a worthwhile car to own, certainly as much fun and somewhat classier than the equivalent Ferrari.
Wayne