Its been quite a few years for me. let me see if i can help you.
Cat back would be a good start. 2.5 inch is I beleive the largest that will fit over the axle from the cat to muffler. There are 2 different types of dual exhaust, true dual using 2 mufflers, and then the dual which our cars are more set up for, 1 muffler layed sideways, single inlet, dual outlet. The factory mufflers are not dual out, but just like the corvette, grand am, CRX, and I am sure there are some others, I have an SLP exhaust for a grand am, my welder needed to shave a couple inches off but other then that it fits perfectly, I cant find it on SLPs website, but I did find this which is a how to for that exhaust
http://www.pfyc.com/supportfiles/ga5004install.pdf
I think mine cost me about 6-700 and my friend installed it dirt cheap, maybe 50-100
As far as a cat, I dont think it is necessary to get a high flow, unless your looking for more performance off the bat, I do not have mine done, becasue I am not sure of yours but it is illegal in Maryland to remmove a cat that is working fine. So a shop may not do it for you.
Borla also makes an exhaust for the Alero, and since SLPs grand am worked, I assume any grand am exhaust could work on the alero with some minor cutting of the pipe. Or you could buy 2 mufflers and do that way.
hope that gets u started, good luck