Unless you find something clean or at least honest with known mods and previous mods that have been returned to stock - It is certainly riskier to get the STI. But no risk, no reward.
As far as modifying the XT. I think stage 2 and some suspension mods is the perfect balance for DD and spirited driving or some autoX. It will cost you less than 3K ( go buy something else or pay off debt or put in your kids college savings). Also some nice new rims and summer tires will get you out of soccer mom status.
Plus you know the car is reliable.