DeadlyNightShadeofViolet
Now I have got that out of my system.
Do not wait. The reason being, that statementing is a 26 week process, with 4 defined stages, and 3 'end points' (consideration of request and decision to assess vs. decision not to assess; assessment; decision to make statement vs note in lieu; finalisation of statement). Obviously, if everything goes smoothly, you will get to the last 'end point' and go skipping into the future...if not, then you need to have time to get the process back on track after the LA have decided to either a) not assess, or b) issue a note in lieu, or c) issue a statement that is unsuitable.
You as your DS's parent, are best placed to choose mainstream or specialist education. I am known (I think) for being relatively pro-specialist eduacation, but actually what I am is pro-suitable education.
In 99% of cases, parental choice for Mainstream must be honoured. It is rare, but possible for a LA to argue that it goes against efficient education of children at the school to give a MS education. It is much more common for a parent to express preference for a special school and be turned down due to lack of places.
WRT the letter, use the template, but your approach is entirely personal.
What I did was to follow the template, then in the 'special educational needs bit' I bullet pointed the different areas of need in the broad areas of:
Medical
Sensory
Fine Motor
Gross Motor
Emotional & behavioural
Cognitive
Then for reasons why the school couldn't cope alone...hmm. What strikes me, is that your DS sounds quite academically capable, but his ASD and sensory traits will stop him reaching his potential. So, you can use the word 'complex' which is quite a good 'tick' in their boxes. So, your DS has 'complex needs' and without support x, y, z....
If you feel he will need 1:1 support you can put that in too.
Bear in mind, that most LAs, in response to a request to assess, simply take a pack and put it in the post. They write to the preschool for their initial views, write to you for your views, and so on.
It does NOT have to be perfect, in fact you could just write 'please assess DS for a statement' and it would have a similar effect. BUT what the letter in this format does do, is send out a subtle message to the LA that you know what you are doing.