We have typical builder-type of AC units on our Summerlin
(Las Vegas) home. I've been told by a reliable source that
they are around 9.0 SEER. Our summer electrical bills are
in the low $700 range per month, or about $500 for AC after
factoring out the pool pumps and domestic consumption. The
two units are in their 6th summer.

An HVAC contractor tells me that we can save substantial
power dollars by switching out the outside units and the
inside coils to Rheem 13 or 14 SEER units. He said that
RHEEM is probably the most "basic" of the newer systems,
still uses R22 for lower head pressures and has more generic
low tech parts than other systems. He said that it was what
he put in his own home. The difference between the 13 and
14 SEER units is the air handler, which he said he might or
might not be able to use until he does a static air test on
it to see if the blower and ducts are sufficient.

I guess my first question is whether RHEEM is decent, basic
equipment. If not, what brand is recommended?

The contractor also looked at the diffusers in our ceiling
and told me that I should change them out with ones that
offered low restriction. He said I should do this myself.
Is this correct to get a more efficient system?

Nobody.