Lease, if it's only for two years. If he thinks he will keep it for future work then look at buying. Only his accountant will know for sure. On the other hand accountants have never liked airplanes. Yearly operating budgets tend to be within 10% regardless of the type of airplane, within a particular class that is.
If you buy one of those $500,000 G2's let me know, I'm current!! 12 passenger seats, only $1.5 million a year in operating costs and in two years assuming the market improves he could probably sell it for $510,000. No crew overnight expenses, do the round trip in one day....What a deal.....
But seriously for $200,000 you are looking at an old 421, 340 etc. Average block speeds are going to be around 180 kts or so and require a stop or two. Salina, KS. and Grand Island NE. are both good. I agree that mountain performance in the summer is going to require a fairly high performance airplane, most of the piston twins are not really that good loaded up. I can see taking off with a load of 6 workers, their bags and you and your overnight bag with enough fuel to make Pueblo. Then you still have a 1300 mile slog ahead of you. Coming the other way factor in 30-50 knot headwinds.
Looking at older turboprops that are not that expensive to buy, but more to operate than a piston and have 6 seats. hmmm. By the way is that 6 passengers plus crew, or 6 total?
Realistically you are looking at a stop or two with a turboprop as well. By the way, the rule of thumb is: If your boss says he wants 6 seats, look for a 7 seat airplane. Those sideways bench seats are uncomfortable and cramped on a long flight. Having an empty seat will work wonders for the passengers disposition. If it turns into an all day adventure he will have a mutiny on his hands.
Look at a King Air E90, F90, B200 and maybe a Cheyenne 2. I've flown the F90, B200 and Cheyenne. The B200 has a bit of an engine upgrade from the original 200, better for the mountains. The Cheyenne 2 is almost a rocket but would be a bit tight on space for 6 big passengers and their bags. The early C90 is cheap, but not all that fast. The King Airs have some great aftermarket stuff available from Raisbeck that would help the takeoff performance out of Colorado.
http://www.raisbeck.com/If it is a two year job be dammmm sure you find something that won't have any major SCHEDULED maintenance for at least that long. Unscheduled is another story, ask anybody who owns an airplane.
I second the motion to try a couple of charters, find somebody with a King Air, Citation 5, maybe a Westwind 2 (not a 1, its a dog). See how they work, these days you could probably find an operator that would love a 2 year contract at a pretty reasonable price. I also agree, the insurance company is going to want fairly high times for this class airplane if you buy or lease. You can get a break from them if you fly with two pilots even in a single pilot airplane, so you may still have a job! As much as I hate to say it, the airlines will also give him a big break if he promises 6 seats per week for two years.
Get an idea of asking prices:
http://www.controller.com/ http://www.aso.com/ It is still a buyers market but I'm seeing some slight signs that it is starting to tighten up.