TAS, What is your market? I have been in the charter business for almost 25 years and that really drives what aircraft you purchase. Is your market price driven? Or is it upper end? Who are your competionors and what are they flying? Are there contracts out there that would justify a particular aircraft? You need to answer these questions first. Next what kind of airports would you be operating out of? What services do they have? What kind of facilities and staff do you have? These are other important questions.
Next forget a Lear 23. I fly Lears regularly and love them. But a 23 is a violation waiting to happen. First of all the newest 23 is 36 years old. They only made 99 of them, so parts are difficult to find. Next the Basic Operating Weight (BOW) averages 7000 to 7600 pounds. Add that to the fuel capacity of 5388 pounds gives you a weight of between 12400 pounds and 13000 pounds. Which considering that the Max Gross Takeoff Weight is 12500 is somewhat limiting. Add to it that the systems are from just a little bit different to very different from the rest of the Lear family. Besides the noise rules in Australia are such that most if not all the Lear 20 series are dead. If you go that class a Lear 35 or a Westwind would be a much better choice. Better range and flexibility.
In the light twin class, I would go with a Navajo Chieftain. One thing I found, passengers dislike climbing over the wing, especially women. The parts are a little easier and less expensive to get than Baron parts. Don't get one with the Panther mod. That mod works well on the smaller versions (C and CR) but hurts the Chieftain. And before any of you experts out there make any smart comments, I also operate a Chieftain with the Panther mod. I flew it before and after the mod. After the mod I considered it the sickest flying Chieftain I have ever flown.
In the turboprop area, the King Air is a very conservative aircraft. The CE441 or the AC690 make good charter aircraft. But I would get one that had the -10 mod done. All the stories you hear about the Garret 331 really only apply to the early engines. Starting with the -10 they started to get it right. I have flown Metro 2 with and without the -10 mod. The difference is like between day and night. I found that I could operate a Metro 2 or 3 cheaper than I could a King Air. Better speed, range and capacity. But you start getting into different rules.
A Citation 500 would also be a good choice. Cost per mile equal to or less than a King Air. A jet.
What it comes down to is what you need. What the market will support. And what you can afford.
Good luck.
Richard