Every one of the 135 ops that I have worked for or run was allowed GPS approaches – we had equip from Beech and Piper. We didn’t have to do anything special to get those other than have the required equipment (nothing special there). However, having an ILS and a 5000’ min. runway would open the door to much more opportunity.
The 135 operators have to plan on more runway than what they need, and this is an issue that on the front burner because of the SWA accident at Chicago Midway. Additionally, in inclement conditions, the 135 operator has to plan on even more distance. With an ILS, you will open the door to more opportunity – take one look at KAAO (Jabara-Wichita) traffic count since adding the ILS, and you will see a nice increase in traffic count.
As to Part 91 operators, I think it will depend on the equipment. Some larger 91 operations run to a 135 standard to get lower insurance premiums. Additionally, if the aircraft is a commuter category aircraft (no matter what part ops they operate under), they have to at least adhere to the higher of balanced field length for takeoff (highest of accel-stop, accel-go, all engines takeoff distance), weight to meet takeoff climb requirements, or brake/tire energy/speed (whichever is highest). If the aircraft is 135 and commuter category, then there are more requirements.
For example, for our aircraft, using your 4200’ runway with a 10 kt headwind component, we would be limited to the following: at max weight, no hotter than 39 C (max weight to achieve t/o climb requirements); tire speed isn’t an issue for us; and, at 39 C, we would be limited to a max t/o weight of 14,000 lbs (900 lbs under max gross) for balanced field requirements (a 5000’ runway would work for our plane).
I don’t think that you could go wrong with a 1000’ extension of your runway, nor do I think the addition of an ILS would hurt either (just make sure it is to Rwy 17 as most of the strong stuff comes up from the south…). Why ILS over WAAS – most charter aircraft are not new, and the FAA will never have a problem certifying an operator for ILS approaches – play to the largest niche, not the smallest.