Smileless and Austin, your situation sounds uncannily like mine, except I didn't go ahead with lending my daughter money in the end. She and BF were going to buy a house with each putting in 50% of the money and DD's share was to be paid by us and protected by a Deed of Trust.
However, I withdrew after BF showed what I thought was controlling behaviour, both towards DD and towards me in the transaction. He was not keen on a Deed of Trust, was reluctant to have a survey, and flatly refused to have any searches done. DD was just happy to let BF be 'in charge', which did not fill me with confidence. His parents therefore stepped in, but insisted on the property being sold and being paid back their money, with a share of profit, two years later.
My relationship with DD has been badly affected by all of this and sometimes I wish I had just given the money, but that would not have been fair to my other DC.
I guess what I am saying is that both in my personal and professional life (barrister), I have seen the rifts that money can cause, usually when a third party becomes involved in the equation ie a spouse or partner.
I am not sure what the answer is, but you are right to be cautious. I would always recommend having clear documentation so everybody knows where they stand. I would also recommend separate solicitors, which gives you the option of saying that your solicitor insists on things being done in a certain way, which depersonalises the situation.
The time to get the agreement in place is at the beginning, when hopefully everybody is clear about and happy about what they are signing up to.