I've used Summit's epoxy and 2K urethane primers several times and have had no issues with them. I used their single stage color too when I painted my truck. Went on easily and flowed well. It was basic white. I believe their paints are rebranded Kirker paint. For filler, all I ever use is Rage Gold. Goes on smooth and sands very easily.
unless you're located in the middle of nowhere, your local body and paint supplier can be a good resource. i never liked summit's shipping policies, costly and takes forever. forget one thing and you pay out the nose. i'd rather buy amazon (and have)
Filler (surface leveling material) varies widely. I, through years of working with it, have found the pricier filler is worth the cost. It is finer and requires less "after sanding" work to get a good surface ready for paint. The less expensive fillers have a tendency to "pin hole". As far as paint goes, use the same product from bottom to top. Every supplier will tell you this. While the base properties might look the same the slight differences can cause problems down the road. As stated before, talk with a automotive paint store and educate your self. Many offer materials that vary in price and final finish.