I just finished installing a new UV treatment unit that I bought on Amazon USA. It is a HQUA-TWS-12. The price was very good especially considering that it came with spares of the two wear parts, the quartz glass sleeve and the UV lamp. This unit is a designed much better than the Mass 8SS that is typically used in this area. It also has a capacity of 12 gal per minute which matches the maximum output of my pump. The new unit is much easier to maintain than the Mass, and replacement maintenance parts are available on Amazon USA. I kept the old filter system. I buy washable filters (a hose and good water pressure are needed) rather than work with a back flush system. I like to inspect the filters and replace them when that is warranted. That of course is just personal preference.
I have done quite a lot of plumbing over the years and for me the installation of the HQUA unit was fairly easy. I wouldn't advise trying it if you are inexperienced.
I hope this is helpful.