David - this is the same question you keep asking, just with a different aircraft type again. In all of the previous posts people have been actively encouraging you to approach current operators to join their fleet rather than launch into the costly, and uneconomic, process of getting your own AOC for one aircraft - operators will be the best source of information on operating these aircraft rather than all of the opinions offered on here (which whilst always useful will also contradict one other).
All of your questions have been answered before, the salary of a Navajo Captain is the same as a 421 Captain. The operating costs will be similar for the 421 and the Chieftain; the Navajo less because of the non-pressurised hull.
There is a Navajo operator at Welshpool, a place you've mentioned before as being a possible base, why not approach them - they never have availability when I want it and so, I presume, would be interested in another aircraft to supplement their fleet. If you can't even do that part of the leg work yourself you're not really doing yourself any favours...
I know Centreline have offered you advice as well as other operators via both public posting and PM's - I'm really not sure what you've got to lose by taking it - other than to keep your name known on here as someone who won't take any offered advice. Which isn't the best way to start your career as a pilot!
The thing you need to bear in mind is this: If you approach an operator for advice, which will likely cost you nothing to begin with, then they will provide you with the answers to all of the questions you have and all of the information you need; if you just persist in asking the same questions on here time and time again you're going to get no replies at all to any of your questions, regardless of the aircraft type you're looking at this week.
I hope you take this constructively, but when operators are actively talking amongst themselves about "the guy who keeps asking all those questions about operating one Golden Eagle" I'm offering you some advice before your reputation preceeds you.