3/4" in and out is standard
If the exsisting is 1/2" you may want to pick
up a book on basic plumbing design and review
your whole system.
CPVC should not be affected by your climate.
Check your local building code on the PVC- cold
water side, some areas allow it, but typically
when plumbing where CPVC is allowed, all lines
are run with it, if nothing more to reduce types
of parts you'll need.