The first step to an ink mixing system is demand and cost evaluation. Do you regularly have requests for custom PANTONE colors? Or colors that you don't regularly stock? Secondly, have you factored out (based on costs of kits and measuring equipment) how much extra you might have to charge a customer to cover the cost (time to mix, ink, etc) of offering this service?
Wilflex has a few different kits (MX, Equalizer, Quantum-One). Do you want a PVC and Pthalate free mixing system? This will cost more than the non-pthalate/pvc compliant MX kit.
I have the MX kit. I never even reference the PANTONE color guides because over the last 10 years I have NEVER once had a color mix incorrectly. Their formulas are fantastic and they offer a software program that instantly tells you what percentage (weight and volume) to mix for each PANTONE color in coated/uncoated. It even does some colored metallic mixes too.
I have a friend who has the Union Mixo-system at his shop. He complains he regularly has to reference the color guide and that the formulas aren't always accurate. Now this is just ONE person and could be his mixes, scale is off, etc. But he never really seemed confident with the system. My MX system really is flawless, and I've done a LOT of mixes over the past few years, and regularly re-mix colors for customers and have never had a problem.
If you run a high production shop, be prepared to buy an ink shaker, which can run you in the thousands. Mixing the ink takes time, some of the colors are thicker than others, so some colors can be harder than others. If it's a smaller shop mixing by hand isn't an issue.
Hope this helps you get started on the way. My personal recommendation is the Wilflex MX system if you're not required to have PVC/Pthalate Compliant inks. If you are, then go for the Quantum-One kit. It's essentially the same thing, but compliant.