Budget is not a huge concern.

I don't think my piping has to be corrected much, if any. Small changes of course to re-route the plumbing but I'm plenty capable.

I do have lots of trees etc (from neighbors yard) hanging over my pool, but I kinda like the shade so it's a catch-22.

I had a DE filter once before. Changing out the DE about once a month is no big deal. Pulling apart the filter and cleaning once or twice a season also doesn't seem like a lot of work for the return. I wouldn't mind going back DE, I just want to make sure that I don't do all the work but still have murky water.

What size pump/DE filter would be appropriate for a 18,00-20,000 gallon pool?

I really like my water clear. It doesn't have to be crystal, but it seems that there is very little in-between. It either seems to be crystal clear, or murky. Not much in between.