I get the big picture- suddenly you're looking at 5 users of the aircraft versus two. Big difference.
In my experience you need to get a grasp on the expected number of flight hours. Anything over 500/year on the aircraft justifies 3 full-time pilots with a budget for contract labor. If you have a full-time technician it would be nice for him to have some part-time help as well (for late night arrival/early morning departure situations). I think its great to have 3 guys with a contract PIC and SIC around; if you go to 4 pilots its hard to justify contract labor.
And which one of those three pilots deals with the schedule? That's usually the chief pilot, or if the department has a scheduler/admin. 5 users is a lot to balance, especially when the holidays are here.
As for the salary issues, I suggest getting a hold of the NBAA salary survey (yes, you would need to become a member and then purchase it). If not, there are countless guys on here with the pro pilot survey numbers. Unfortunately, I'm not one of those guys. Sorry.
In a nice presentation, the owners have to see the dollar amount of investment that goes into initial, recurrent and ancillary training each year (RVSM, P-RNAV, Med-Aire etc). They need to understand that lower salary leads to turnover which leads to a higher budget at the end of the year regardless. If they get it- great. If not....good luck.
And lastly, there are many 'consultants' which will charge a lot to help set up a department. Some are great, others are bottom feeders. Be alert.
Let the fun begin! This could be a great opportunity.