Hi Neil, I have a single 500 series Racor and it's a cinch to change filters: unscrew the T handle (which in my case has been replaced with a gauge), lift off the cover, old filter out, new one in, add a bit of diesel and put the cover back on. It's so easy and usually mess-free that I personally don't feel the need to have two filters, but I'm sure others feel differently and prefer the convenience and safety of a quick change out. I did install a polisher system last year, with its own Racor 500, and I'm glad to have done so. I can polish fuel in the tank anytime it might need it.

Not sure about the buy vs make … I'd probably buy the finished unit but probably not that hard to pipe your own.

Max