I just looked up Kelly Blue Book values for you and you should be able to get a used one anywhere from $15k-$22k depending on mileage, condition, transmission, & whether or not it's a certified pre-owned vehicle or not. Certified Pre-Owned Vehicles are definitely your best bet when it comes to buying a used vehicle. You might pay a little more in the beginning but it's worth the guarantee that you won't be paying a whole lot more in the long run.
As Rockin' Blue's stated, they are a great car to add mods to & are a lot of fun to work on.
And yes, you will enjoy the car as it is relatively easy to make modifications to. I own one and it is a blast. I like the body style more so than the newer models.
The only thing with the 05's is they all or most came with 17" rims - most brake upgrade kits require you use 18" or larger rims for clearance sake - but don't let this stop you from making your purchase.
B302 5.0L, Saleen SC w/Extreme CAI & Brenspeed tuned SCT X3 programmer, Shelby GT500 Fuel System, Bilstein Struts & Shocks, Steeda HD Belt Tensioner, Fluidyne Alum Radiator, Stewart EMP High Perform Water Pump, Steeda Front & Rear Sway Bars, Steeda Lightweight Radiator Support & Billet Rear Control Arms, Hawk Brake Pads, Goodridge SS Braided Brake Lines, Powerslot Rotors, Powerhouse Automotive Alum Driveshaft, BASSANI Aft-Cat w/x-pipe, GMS Coil Packs, LPW Racing Differential Support Cover...
Recommend getting prices from KBB, Edmunds, and several auto marketing sites(Auto Trader, etc.), and then using the average price from all of them as a starting point for negotiating. Otherwise, you may end up paying too much.