My wife and I used a family friend as a donor when we had our daughter, who is almost 3. (Sorry for the wall of text.)

• Nothing unexpected came up for us. The donor and his wife treat our other 2 kids (from my wife's previous relationship) like their grandchildren and they see all 3 kids fairly often. The donor has never had interest in having his own children, he already had 2 step children. Both the donor and his wife saw it as lending us something simple, like ibuprofen. Nothing they were emotionally attached to, and I think that is a huge reason why we haven't had any issues.
• There is no visitation set in place, they see her when we have get togethers but thats it. He isn't her dad, he is just a donor. She obviously doesn't know it though, and we haven't decided if we will tell her when she is older. The donor and his wife love our kids but in a grandparent type way.
• We did not do any genetic testing. We knew the medical history from both myself and the donor and didn't see anything to warrant any additional testing. They do some general testing in any pregnancy and thats all we did. Thankfully the biggest "health" issue is horrible vision, which is correctable with glasses and not something they can even test for.
• If we were to do it again (and we will), I don't think we would do anything different.

We have a beautiful daughter and were able to use someone we trust without having to worry about them feeling like they are a parent to her. That was our biggest concern. This may be different in your situation, but when we decided to have a child we were having OUR child. We didn't want to have a child with someone else, our child would have 2 parents and thats it. I'm sure you already have, but you need to make sure you and the couple you're providing for are completely clear on what exactly your involvement in the child's life will be. If you will be in their life, there should be clear boundaries regarding the relationship with the child. I think what you're doing is an amazing gift, and you're an amazing person for helping the couple that's asked you :).