Swaybars, bushings (prob. worn on an '05), those are the first things I'd do for handling, then struts/springs next.
For performance, replace the downpipe and uppipe with high flow versions, along with a stage 2 tune.
First and foremost, make sure all maintenance is done first. Obviously all fluid and plugs; and if you're over 100K make sure the timing belt and water pump have been done.
Modding is a slippery slope with these cars, good luck.