all depends on the type of flare.
http://www.truckcustomizers.com/images/prods/BUS/Bushwacker-Flat-Style-Fender-Flares-Jeep-Cherokee-XJ-large.jpg once I finally do my lift I would love to puts these on my Jeep I just cant see spending $500 on them.
As for the undercoating do it my Jeep was done by the original owner and there is not a spot of rust on the undercarriage. as for aftermarket not sure whats available Best thing I can suggest is to find a eterra forum and see what they recommend