Hi. You've got the pre facelift model (pre 06) which has the different lights and dash to the more common facelift model.
I think the 3 splits in the radio can be cut through to create room for any double din stereo.
I like the prefacelift headlights but I believe they can easily be swapped for the ones from later cars (the one with the hump over the sidelight bulb) and aftermarket units are available also. The rear lights are a bit harder to swap as the loom is different I believe.
Good cheap mod is an induction kit but it depends if you like the extra noise when accelerating. Lowering springs if you like the look and can sacrifice some comfort. Exhaust manifold gives a bit more torque but they are more expensive and fitting is a bit hard because of access, or you can just change the manifold flexi pipe for a bigger bore one if you only want to do a small mod and save money. Some good brake pads would give better brake feel without "braking" the bank. Cat back exhausts arent too expensive particulary if second hand, although they're not a well insulated car so can be noisy inside.
2005 Fiesta ST
Newman fast road cams, Fusion Fabrications custom inlet manifold, custom exhaust