I know this is a few days old now, but hopefully OP is still around!
Is the company your DH is going to be working for a larger business? If it is, then the decision on whether a contract is inside or outside IR35 lies entirely with them.
The good news here is that it appears that this is the case and they have decided that it's outside.
This is good news because if HMRC do decide to challenge that decision, it's the employer they'll go after, not your DH, and therefore you can do whatever you'd like in terms of his earnings.
How long is the contract for?
If it's only a 6 month contract will no scope to renew, it might be an administrative hassle in setting up a company to save some tax.
If it's either a longer project, or is likely to lead to further similar contracts (though each contract is reviewed by the client on a case by case basis, so it may become an "inside" contract later), then having a company is going to be better.
We recommend to clients that they take a salary equivalent to the Employers NI rate of £175 per week (so £9,100 per year). At this rate your DH would be "paying" NI but at a rate of 0%, so is still earning a State Pension credit.
Anything over and above that that he takes from the company is treated as a dividend payment and subject to those rates instead.
This set up used to be very, very tax advantageous, but with the increases in Corporation Tax and tax on dividends this advantage has narrowed significantly.
On a £700 day rate though, the earnings are potentially significant over a longer contract, so it would be beneficial still to operate through a company.
I would recommend engaging an accountant to provide you with some formal advice and to help set up the company etc.
Your DH will also have to be wary of the VAT registration threshold. At £85,000, he'll only have to work just over 5 months before he hits it.