I know it wasn't my task but I've been thinking of some safeguards and restrictions to put in place to make as fair as possible.
First of all yes, mandatory , independent and confidential psychiatric assessment prior to insemination, after ,all throughout the pregnancy and at least a year after birth. If things go wrong interventions and options to be offered.
The parents have no legal right to the baby or the mother up until birth. So the buyers can't dictate what the mother eats, does, what medicine she might need, they have no rights to decide if complications arise or any health issues.
On the money side, while they can't pay "for the baby" costs for the mother should be covered with documentation (time off work if unpaid,medical costs etc). They should also be responsible for any medical (mental or physical) costs as a direct result of the pregnancy/birth. The parents are legally responsible for this regardless of the outcome of the pregnancy. In the same manner they are financially responsible for the baby even if they don't want it. Any and all associated medical costs , finding them a new family , being looked after the state etc.
Checks on the parents , DBS , a psychiatric assessment for themselves , who are they,where do they come from,where are they going etc. No opportunities(or as least as possible) to just "disappear" in the ether.
The mother has the right to change her mind either during pregnancy or up to a year after birth. This is controversial and it has it's own downsides so I have to think on it a bit more.
The mother has her own panel of "support". Independent Doctor,lawyers ,maybe even a social worker to advise about any and all risks.
The option of being in the child's life is they so wish, unless that would be detrimental to child as assessed by an independent professional.Same for the child, full access and information to any and all information they might require, including the possibility of contact if they want to.
In case of death the baby (or the mother on their behalf if she chooses to keep the baby) is a beneficiary of the estate.
I'm sure I missed a lot(which shows how complex this is) but that's just the very least that is needed.