MOCR wrote:
... was this a mod of an old airframe, or a completely new bird ... it seems most would be past their usual service lives, and if this was a mod, why they chose this variant over others
For the application, carrying 8 or more sightseers at once, there are no cost effective modern airframes I'm aware of, and modifying an old S-55 would have been fairly inexpensive (compared to buying something new). Airframes aren't what's life limited (look at at how many DC-3s are still putting in 10,000 hours years after year around the world), it's engines and rotors that have life limits. You can bet that 5 blade rotor is new equipment all the way through the transmission to the engine, and the turbine is, of course, new as well.
MOCR wrote:
... is the engine larger compared to the radial? Aside from aerodynamics, I'd like to know if the lengthened nose was for more room for the turbine, fuel or cargo deck?
The nose in lengthened for one reason: weight and balance. The turbine is both smaller and lighter than the original radial; it has to be located further forward to preserve the helo's CG. Look at the pictures of S-55s in airliners.net - there are several pictures of turbine helos with their cowlings open so you can how small that turbine is.
MOCR wrote:
... vents just below the cockpit are the intake for the turbine?
I assume that's correct as there aren't any other openings (unless the intake is taken from below, but risk of FOD (foreign object damage) would argue against locating a turbine intake close to the ground).