If you knew he hadn't had sex with her (or any other prostitute) aside from the STD issue, would you really feel much better? The truth is it is unlikely that you will ever know for sure without him being honest with you. How likely do you think this is? His soliciting is bad enough alone as is his complete avoidance of the issue.
His sexual needs appear to be a big issue here, they seem to be pretty extreme, is using a prostitute his 'guilt free' option for getting these needs met. Does he feel guilty for 'inflicting' his needs on you? I am in no way implying that this is your fault btw.
Because of your financial problems it seems the only way to make any progress at the moment is to see a marriage counsellor and perhaps some individual therapy for him. How you can convince him to do this I don't know because it seems pretty clear his ploy at the moment is to deny everything no matter what evidence you come up with.
I really feel for you, I found out a few years ago that my SO was contacting prostitutes and was using porn very heavily. Although I tried talking to him about it, he always completely denied everything, even when I shoved the evidence in his face. It took a year of separation and counselling for him to get us back on track. Rebuilding the trust is still ongoing but improving all the time. It was painful and hard for both of us but it seems to be working (fingers crossed!) My SO only agreed to counselling once I had thrown him out, it worked for me but it's a dangerous move, sometimes once that is done it's difficult to go back.
Good luck with whatever you decide to do.