I offer the following advice:
1. Decide what size and type tires you want. 15", 17", 18" etc. Determine what is available. Do you care if matched front and back are not available? That starts the process.
2. Decide what wheels you want to match those tires. Determine the right offsets for your car and the look you are trying to achieve. My Gp. IV has a 13" wide rear wheel with a very small backspace. There is only ONE tire that fits it available in a Z-rated compound. They cost $700 each. Still want to proceed?
3. Most importantly, do NOT cut and install the flares until you have purchased and mounted the new wheel/tire combo, set ride height/stance, etc. If you do, you will be chasing your tail looking for some wheel/tire combination that is unubtanium. Also ensure if you are lowering the car, now you have to consider new springs/shocks.
Flaring the car is not trivial if you want it to look right.
The only difference in this process is if you elect to use fiberglass Gp. IV flares from the factory. They are what they are and fitted to the body. Even then (trust me I know) the fitting is alot of work to get right. The downside of these is you can not alter the fit to match wheel/tire stance as they come with the flange molded on so unless you want to build your own flare, you are kind of stuck.