Fenders are thin and will flop in the wind when you are driving. Quarters, well you don't have much choice as i have seen very very few NOS ones that are $3k+ each. The new repro door shells from what i have heard are really nice.
Get original fenders (nice ones on ebay can be had at times cheaper than repro pieces), repro quarters but expect some work getting them to fit properly, and either used or new repro door shells. Rust free door shells are not cheap though, all depends on the budget but nothing fits better than ford pieces. Oh and i have heard many people disgusted with the repro rear valance panels. I guess those pieces are waaayyyy off.
My car is in the process of having pretty much the whole passenger side replaced and it is all being documented so depending on your schedule you'll be able to see for yourself in another month or so.
I bought the full face quarter skins for a 69-70 fastback. The ones I bought are from NPD and I have found that they are pretty far off of what reality is, but with some patience, skill, and ingenuity you can make them work. For a 69, the curve below the scoop is not shaped properly and will have to be formed with a hammer and dolly. The skin is overall about 1/2" longer than what is on the car, so it will need sectioning lengthwise. I ended up sectioning behind the scoop on both sides. This allowed keeping the door jamb untouched and factory appearing. The sectioning on the rear is being done about 1/2" in from the rear end caps to hide it as much as possible from casual viewing. I am planning to butt weld the top seam near the belt line (the skins were too tall that direction too). Either way--lots of work to make it look good. I did have good luck with the outer wheelhouses and trunk floors also purchased from NPD--they dropped right into place and look great. A few minor variances from OE in appearance, but really close.
I bought panels from five different suppliers in the past two months and BY FAR, NPD had the nicest stuff. Mustangs Unlimited had the worst. We should all be thankful we aren't into Torino's cause it's amazing how cheap we can buy this stuff. Love these Mustangs.
Also, rear valance, taillight panel, floor panels, quarters, fender patches/braces and rocker panel were spot on. Only problem on my 67 was outer rockers.