I don't know whether your GT is an oil burner or not, but the 2.4 is basically the 'big brother' engine to the 1.9, so the same common rail diesel issues apply, i.e. importance of cam belt servicing, EGR valve issues etc.
Unlike the GT, the 159 also has a DPF filter, which (like most DPFs) is prone to clogging up if your driving involves lots of short runs. As always, diesels are much more appropriate for long motorway schleps rather than around town. If you do low mileages, or around-town running, then diesel may be a false economy.
The 159 is a much heavier car, and the 2.4 is a bigger engine, so don't expect great fuel economy. Budget for mid 30s mpg.
The TI is more than just the wheels. It's lowered on uprated suspension, with side skirts, brembo brakes, different interior, more toys etc etc. But don't let that stop you getting bigger wheels
. Be aware that the 19'' tyres can be expensive and give a harsher ride. 18'' horseshoes or ten-spokes may be a better compromise.
Otherwise, check the steering, as 159s are prone to rack failures. Check for noises when going from lock to lock. After you've got it (i.e. it should be a showstopper), check that the plastic coolant bleed valve at the top of the engine has been replaced with a metal one (do a search), and that the power steering fluid is green rather than red. (do another search