I think I need to replace my combo electric/gas unit. It is a Rheem and has worked well for 18 years so I think I should get a new one before it fails. Is this a wise move or should I just change out some key parts? Also, what SEER level do you recommend - 13, 14 or maybe 16? What brand is the best - Right now I have a quote on a Trane XL-1400 and a Ruud RRNA-B. Look forward to your input.