Engine mods - depends on the state your in, you should be more specific. SA has no limits, which is where i am, so i cant help.
Body Kit - IIRC all pintaras have the same base front bumper bar, and if you buy the trx front lip its screws into the front bar. Side skirts also fit, but to have the sideskirts you need to have the mudflaps, all screws in aswell. Rear bar probably the same, but im not sure, so someone else will chime in to help with this
as for other kits, subaru wrx 97-00 front bars apparently fit with extremely minor modification, and many others can be made to fit with a but more modification eg c33 laurel, r33 skyline.
Price estimate - depends on how much u can get a second hand trx kit for. some people sell for 75-100, others want 3-400 for a kit.
if you go aftermarket wrx or whatever, your looking at much more.