If you can do basic plumbing, you can probably replace the system yourself. I would recommend avoiding the companies that are pushing only price. Buy from a company that has a real location, not a PO box, a UPS store, or from a house.
Installed prices in the 1500-2000 with a RO, the liability insurance, the licensing, certifications, etc are worth it in my opinion.
Are you on a municipal supply or a well? What are your basic water conditions?
The Fleck valves are the most popular with the commercial and industrial guys, the same design goes into their residential systems.
They are very easy to service and maintain, but the best part is, they rarely need any service. See my link below, it is a short video on a complete 7000 teardown.