I know that lots of people say "just get started as it takes ages" but it really paid off for us to go a little slowly and write the best application that we could. It you start now, you could send it in before Xmas when relatively few other applications are being sent in. I think that was one of the things that helped us - we were told our son was too able to qualify for help and you seem to have much more evidence of need than us by the sound of it. If you can get through the first hurdle which is "Agree to Assess" you have a very high chance of getting an EHCP in place and so this is by far the quickest way forward (appeals could add at least a year or so to the process). I highly recommend going on an IPSEA Law foundation course - there are some coming up this autumn and you can also do it online.
They key thing is to look at what evidence you have. A diagnosis report? Any S< information? OT? Nursery Reports? If you don't have much, then concentrate on that to start with. Can your nursery help by getting professionals to come in to advise on what he finds difficult and what "provision" he might require. What you want them to describe is a list of his SEN (listed our under the 4 Areas of Need from the SEN COP) and the a list of what things should be put in place to meet that need. It is that list which will help establish what sort of school placement he will need so it is worth getting all the information that you can. Try your local children's centre - sometimes you can do a drop in with an S< and take your child along. You may need to arrange for an audiology review to check his hearing if that hasn't happened already. Your LA Ed Psych service might offer drop in sessions where they could advise on the type of provision which might suit - although they don't tend to do a proper assessment. Try and find out who your local inclusion service is and try to speak to an ASD specialist teacher for advice on what might be needed.
You are looking to establish a list of "behaviour indicating un-met needs" in the nursery/school setting and how his SEN "limits his access to the curriculum".
Keep notes of all meetings, discussions and phone calls for use in your EHC request. You can make notes or diary entries about different events - eg uncooperative behaviour, signs of stress, . If you then type them up and add a date, this can be included in your evidence bundle. It also works for phone calls and meetings with different professionals.
In my request letter I downloaded the IPSEA template request so I had the proper jargon at the start and end but I enlarged their single page to 4 pages. I started with my son's name, age, school, year and diagnosis; when the diagnosis was made and by whom. Then I made a list of all the professionals who have seen him; then a list of the referrals we were waiting to come through. I also included some other names of professionals I had consulted about my son but who hadn't met him. I then said he had SEN which limited his access to the curriculum as set out under the 4 areas of need (insert list). I then wrote 3 or 4 paragraphs covering the main problems that we had had which told his particular story. I tried to cover the biggest problem first and then lesser issues in order of importance. I thought about how I could best tell our story to a stressed/overworked LA staff member and make it easy for them to understand and to Agree to Assess. At the end of the letter I used the IPSEA paragraphs about the legal test for SEN to show how he met the test; and the evidence I had to show he would benefit from an EHCP ( I asked a professional to write in support of a plan so could use that letter). 4 weeks after the application was sent I followed up to say I was looking forward to receiving news of Agree to Assess as I was sure they had seen page xx confirming a delay in yy and page zz where the professional supported an EHCP. I wrote it in such a way they could cut and paste it into their paperwork. It worked - they said yes so we didn't have to appeal. I still can't believe it but I really recommend getting to understand the law and then being very clear about how your son meets the criteria, with evidence to back it up.
Sorry for rambling but I'd be happy to look at your draft request if you PM it to me at some point in the future. Good luck!