"...before first conversation with a solicitor"
There is a body called the Society of Trust and Estate Practitioners (STEP). Solicitors that have qualified to become members of this society really do have a great deal of knowledge and experience in the area of trusts. If you google something like "STEP solicitor [name of your town]" you will get a list of local solicitors.
The advice will be very specific to your own situation, including if either of you will likely leave an estate that will have to pay inheritance tax (IHT).
For example, if IHT might be an issue then it may be beneficial to set up a trust for your DD with SEN before you die but not necessarily for the other child. If IHT won't be an issue then you don't need to consider this.
"...she probably won't ever be financially competent."
It all gets very complicated and you need advice tailored to your particular situation. You would each individually leave any assets to your DC and likely leave any assets to your DD with SEN in trust.
If your DD qualifies for certain benefits such as PIP or DLA etc then there are various tax advantages for the trust.
In addition, anything held in the trust will not be counted when she is assessed for any means tested benefits.
You will each leave assets in your will separately but you can both nominate the same person or people to act as trustees. So there will be two separate trusts but the same people will be responsible for both of them (if that is what you want to happen).
The best thing to do is to explain to the solicitor in simple language what you want to achieve and they will then go about making that happen.