standbyu gnerators.

I'm thinking of installing a standby generator.  I have propane
available, so I'm looking at a 17k, 20k or 22kw generac unit.  The
22kw is 1800rpm and liquid cooled, while the others are air cooled
3600 units.  The 22k is significantly more expensive, needs a concrete
pad, etc.  I suspect it would last longer.  My question is, does
anyone have an idea of the life expectancy of these units?  Can the
engines be rebuilt if they get worn out?  I doubt this would happen in
a standby application, but you never know.  I'd also guess that the
bigger unit if not quieter, might have noise that is less
objectionable than the smaller ones.  I don't mind spending the money
if that's what it takes, but I don't want it to be wasted either.  I'm
going for such large units because I want my central ac, refrigerator
and freezer to be powered and i want the whole proces to be idiot
proof.