The wheels are a big weakness, as is the gearbox - look, I'll be kind and swap them over for mine if you want to be saved from future issues.   Seriously though, I haven't heard too many people make too many complaints about the 07 STi. Beautiful cars, sure you will enjoy it.

Congrats on picking up the Liberty. They are an excellent car off the shelf but weakness are as follows:

Stock sway bars way too small, go with HD Adjustable whiteline or cusco sways front and rear if you can afford it.
Stock sti pink springs are very bouncy and will give you the shits with the bobblehead action when hitting bumps.
Stock exhaust is very restrictive and the "sti" quads are baffled to hell and quite tinny and cheap if you look at them closely.
Stock tune is retarded badly and tends to be on the rich side, along with the dreaded drive by wire throttle stutter. Flash tune will fix this and give you some extra torque and hp.
Stock rotors and pads for your brembos are average at best, do not buy replacements from Subaru either unless you are prepared to bend over. Go aftermarket and you will see a big improvement in braking.

whiteline HD sway bars will cost you about $250 each plus you will want to get a good wheel alignment at the same time, you can run 1 degree of neg camber on the fronts.

If you want to do the exhaust but keep it quiet with the sti quads then not sure there is any point in changing centre pipe and def not the dump pipe. Hitech in Sydney would be my choice for s/s cat back or just mufflers. Custom dump pipe would be my choice.