I have been happy with Unicolor C41 powder kits. I used a Rollei Digibase C41 liquid kit with separate bleach and fix and got good results as well but now all their kits use blix (at least in the U.S., Freestyle) so I went back to Unicolor. The only modification to Unicolor is to add 1/2 teaspoon of PhotoFlo concentrate to 1 liter of the stabilizer, and repeat every 8 rolls, to eliminate water streaks. I would try Kodak Flexicolor if the the sizes were smaller and it was less expensive.
For RA4, I use Kodak Ekatacolor chemicals: Kodak Ektacolor RA Developer Replenisher RT to make 10 liters (Unique Photo #EKY8415580, $14.26); no starter needed; Kodak RA Bleach/Fix to make 10 liters (Unique Photo #EKY6601629, $14.75). These concentrates have expiration dates on them. See Kodak's J-39 publication for use.
I got boxes of old paper with a couple of enlarger deals but ended up getting rid of it all. Establishing enlarger color filtration settings for your enlarger/film/paper combinations is the only time consuming part of printing and having to constantly change them because of the color shifts that occur in old paper is frustrating. Fresh Fuji Crystal Archive II gives great results (and cold storage keeps it fresh).
Pushing your C41 kit way beyond the storage time and number of roll recommendations can produce negatives that look OK but have enough color shifts to require filtration setting changes when printing. I get 20 rolls from a Unicolor 1-liter kit (vs. 8 rolls recommended) but I develop all rolls within a few days. This produces consistent negatives with respect to printing. So you can push your C41 kit to some extent but it's not worth doing to extremes, as some people online do.