HS-125, 731 engine conversions...
I flew a number of HS125- 400s with 731 engines on them. Each one was different in quality. It all depended on the cockpit layout, AD mods, avionics, and interior. One looked just like the HS125-700 FSI simulator, I couldn't tell the difference from the cockpit. The AD mods can be expensive, RVSM equipped is a big issue if you want to go above FL290.
Frac Capt has posted a nice link an an HS125-400. I liked flying the airplane, stand-up cabin, but the older HS125 still have the honey buckets that must be disconnected and hand carried from the back of the plane and out the FWD entry door. One spill and the carpet is ruined.
One mechanic told me that for every hour the plane flew, it required almost 4 hours of MX on average. Of course, he was the guy billing the operator. Could anybody else chime in on the MX cost of an older HS125?