there are general guidelines but every one does it different. thats what makes the car your own. for exxy, my plan is this (performance only): finish zorst, r34 smic, boost control and guage, safc, fuel stuff etc. some ppl may get the safc first or boost, or something completely different. the best advice i can give you is this - research research research, on sau, other sites, other owners, anywhere. know what a mod does to/for your car b4 you do it. that way you are less likely to do damage, get ripped off and waste cash
good luck man