So firstly i'd say payment for access isn't a thing that should be done.
He should have access, I don't think you can really stop contact unless he's a danger.
That said, he doesn't just get to walk back in after three years and get everything he wants.
Start slow, as slow as makes you comfortable and build up. If it's too slow he can go to court but going to court and saying I haven't seen my kids for 3 years and now my ex is only giving me limited access is going to sound stupid since courts would start slow too.
I'd say start with a talk with DC, see how they react. If they're excited then maybe you can go faster, if they don't remember him or don't seem to care go slower. Don't get their hopes up though, be careful with phrasing.
Contact centres are a good start, video calls, supervised visits in a park maybe or even potentially a letter first. I don't really understand the whole letter thing but it is the sort of step they start abusive parents on when regaining trust.
I started with supervised visits with my DD and I was there from birth. Contact centres were mentioned but weren't appropriate but it's genuinely the sort of thing the courts would go for so I think that's your best bet.
You might have to pay for contact centres since it's not through court (maybe you do if it's through court too) but I would suggest that he pays those fees too. I think that's fair.