Here we go -
1. Can you recommend a suburb within say 20 minutes of Findon which would be a nice "family" area? Want somewhere like Norwood, without the Norwood price tag (am I dreaming?)
West Lakes is very nice. Findon is in the western suburbs so the beachside suburbs (Henley Beach, Henley South, Grange) aren't far but housing there can be very expensive and may not meet your needs. Glenelg and Glenelg North are about 20 minutes away but can be very expensive.
Further up on the LeFevre Peninsula there are some very nice houses in the suburbs of Largs Bay and Largs as well as some at Semaphore. North Haven also has some nice homes but is further north on the peninsula.
If you were to select some houses you liked in suburbs not far from Findon at an online site the Adelaide people here would be able to brief you on the suburb.
2. How accessible is Findon by public transport?
Accessible only by bus.
http://www.adelaidemetro.com.au/ Findon is too far from the Grange railway line.
3. Is it totally unrealistic to live in Belair and take under 30 minutes to get to Findon by public transport?
Yes. The train from Belair to Adelaide is good but getting by bus from Adelaide to Findon would take quite a while. The Adelaide metro site has timetables on both.
4. Can you recommend any pre schools/ ELCS from personal experience?
Sorry - no kids so no experience.
5. Can you tell me a suburb which is in 15 minutes drive of an outstanding private hospital with paediatric A & E...DS has severe allergies, and so this is VERY important
The only hospital I know of which has paediatric services is the Adelaide Women and Children's Hospital situated at North Adelaide.
http://www.wch.sa.gov.au/
http://www.anzics.com.au/paed/about_anzpic.htm
I included that second link because it sort of explains my response to your question 5. Again not having children I am out of the loop a bit but I believe the ACH as well call it is the single service. Having said that I'm advised the ACH is a very good establishment.
Re housing. It's always a very personal choice but the suburbs I've indicated are very nice areas. Anyway this is Adelaide, we don't have too many "bad" areas but if you pick a suburb some of us will be only too happy to give a local view.