If floorpan rust expenditure is putting you off the 156, I'd be cautious about getting into a GT 3.2. Cheapest and simplest option will be to weld up the 156 - if you take out the relevant bit of the interior, then all you are paying for is welding - which will not be expensive.
My GT has been utterly reliable for 4 years - but I did get it with a blown engine, and I rebuilt it. You may be lucky, but a medium mileage V6 can chuck you a £1K bill as soon as look at you.
Jag X300 or similar? Really? In the same paragraph as a 156? We got one for Mrs rxe (had a foot op, needed an auto), and honestly, it is a barge. If I had 9 points on my licence, and really needed to not get 3 more points, this would be the car for me. It is so big and so ponderous that doing 40 in a 40 is pretty scary. The 3.2 used an awful lot of petrol (more than the GT) and I couldn't for the life of me work out what it was doing with it. Good looking thing, but not much fun.