I'll divide your question into 2 questions, because I am not sure which one you're really asking :-)
1. Should I hire him despite the costs involved? You seem to already agree to this, based on what you wrote ("I am willing to set aside...assuming I see some ROI...").
2. How to I justify his high salary to other employees?
A few options:
a. You don't tell them.
b. The CEO's compensation can be structured in a way that his 'salary' isn't that high (and this is what the other employees will see - so no problems), and he gets a large bonus based on results (which could be measured within 6 months and then 12 months). This is also better for you seeing that you don't know how good is will be. Additionally, if he is so confident he can do the job, he shouldn't object to that structure.
c. Make it clear to your employees, that if he manages to generate more income for the company, they too will benefit from it (end of the year bonus/increased salaries/holiday gifts?).
d. the tough option: tell them that if you don't make some hard choices, there won't be a company for them to work at down the line (due to the yearly decrease in profits)
It's your company (which you seem to have managed successfully), so I am sure that you know what would work best with your employees.
I'm happy to help you construct the employment contract with the potential adviser.
Good luck
p.s.: let me know how things turned out. I'm curious :-)