I have some extra live rock 'cooking' in a 40 gal tub with a Koralia 1 powerhead for flow. Is the K1 enough, or should I use a K 3? & should I do water changes with my old salt water?

It makes no difference about water flow. Where is the "old" saltwater coming from?

?

Probably safe to assume from an established/running tank.

Water from my established 37gal when I do water changes.

In that case, adding "old" water is a bad idea. There isn't anything special about used water except that things that accumulate, like nitrates, are often found in noticeable concentrations. "Unused" saltwater is fine.