Another vote for Paula Hall's book. It's been my go-to resource as I've tried to get my head around my own situation. She puts porn addiction on the wider spectrum of sex addiction. Internet porn can act as the gateway drug for other behaviors like webcam sex, hookup apps and 'paid for' sex because it's all just a click away. The internet makes it easy for 'Mr Nice Guy', he doesn't need to go cruising a red light district any more. Hiring a hooker is now as easy as ordering pizza.
As far as you're concerned, all bets are off with him now. You should quite rightly be able to go through every atom of data on his computers, phones, iPad, external drives, USB drives, camera memory cards, phone memory cards, old phones, anything and everything that stores data. It's not 'snooping'. He has had his chance and he chose to lie. He chose to pay a prostitute to tug away on his dick, give her cash so he could shove his dick in her mouth, and then come home to you and say 'I love you' and let you believe that he's the faithful husband. Well, he gave up his right to privacy when he pissed all over your wedding vows. Go ahead and do what you must. Nothing, and I mean NOTHING is off limits now that you know what you know. Paying for sexual services is crossing the threshold into another domain. The underworld of prostitution. So do what you must. All you ever wanted was the truth.
At this stage, does it matter so much about the EXTENT of the habit, knowing that he's admitted to paying for the 'entry level' sex acts. If I was in your situation, I don't think it would matter if it was "just" a hand job or whether it was penetrative sex. It wouldn't matter if it took place in a massage parlor, a hotel room or in his car. It wouldn't matter if he paid 10 Euro or 200 Euro. It wouldn't matter if it was a street prostitute or an 'escort'. Whatever the gift wrap he might invent to make it sound less sordid, it's still paying for sex with a prostitute. That's my attitude anyway. But if I was ever to find out my husband had done what your husband did I would have to seriously rethink my future with him. I've actually been reading about prostitution and it's nothing more than renting the right for a man to force sex on an unwilling woman. The only thing that makes her do it is the money. She wouldn't have sex with any of those johns if there was no money in it for her. Prostitution survivors tell a very different tale from this pornified happy hooker fantasy shit. That's why they end up damaged or dead before their time. It's the most selfish, self serving, egotistical thing for a man to do. Renting a human orifice to ejaculate into.