To answer the OP's question, our reason was that 'we can'. We had space in our house because most of our adult children had left. We had both committed to retiring in the year we did our assessment. We had caring experience (schoolteacher and church pastoral leader).
This seemed to be enough to trigger the assessment process.
If you pass the initial one hour assessment, you will be given an assessing social worker. Our programme was about 10 weeks x 2 hours, when the ASW goes through every aspect of your childhood, family values, relationships, support in the community etc.
It may be some things you have shared here will be red flags, and they will either terminate the process, or help you deal with them. There is absolutely no shame in putting yourself forward, but, as you have seen here, it is important to give the right impression.
Your 6yo would be 7 before any placement, which is cognitively a step change. However, I agree with many of the comments here that she is still quite young, so it might be an idea to wait until she is a bit older and a bit less dependent on you, eg when she goes to secondary school. Depending on the foster child, you can have a lot of appointments, so being able to work can be difficult. They don't tell you this on the process.
Not all FC are high needs. Our 6yo has no additional needs and her school is 5 minutes away. We have found that you have to be very careful when deciding to accept a child - that they will fit into your family. Your DD's needs come first.
Good luck in whatever you decide. You might be grabbed by the social workers, but make sure you are totally happy with any placement agreements you make, and ask lots of questions. We were not happy with fostering placements until I learnt to ask, after telling me they loved sports and baking - now tell me the bad stuff.