On the western slope of the Cascades they don't crud up very quick at all- take that one off the checklist. ballvalve's "bottled water quality"== added minerals for fresh taste.
And if you run it at 110F instead of 140F, even in bottled-water country it can go decades without appreciable loss of performance, but not in TX. 140F would be a tank heater reference point, and you'll rarely need more than 110-112F out of the tap, so when it doubt, turn it down.
SFAIK recirc doesn't void the warranty on the TK3 as long as it meets their minimum pump sizing specs. (Scoll down to the discussion near the bottom of the page.)