Used airliners are the best source of tankers. Screw new.
Yeah, the idea worked so well for the USAF's E-8C JSTARS fleet.
Do a little research on what refurbishing them - not counting the fancy electronics, just washing cow sh1t out and corrosion repair fixes - cost as compared to new builds.
Not to mention the unexpectedly large O&M funds in keeping a "used airliner" in service or the fuel burn on those "cheap" engines.