I just estimated a new AC/Furnace system. One of the problems is the model # for the furnace does not show up on the Rheem website. It is RGPH07NAMGR. I'm concerned that this is an outdated model?

I have a 1700 square foot house. Living levels on levels 2 and 3 and garage down below. Southern California. High ceilings in living room on level 2. Flat roof and the 3rd story never stays cool enough. I'm only going to be in the house another 5 years or less, so I asked for a 12 Seer 4 ton, Single Stage 80% Furnace.

I think I'm going with Rheem because of the size of the Condensor. A tankless water heater must be installed above the unit and this short size fits the bill.

RAMC048JAZ for the Condensor, RCGA48AIGH21BI for the Coil and RGPH07NAMGR for the Furnace.

Any thoughts from anyone? How can I tell how many BTU's I need? All suggestions will be most appreciated.