Fully agree that if you are going to make major changes to diet, you should:
read up on it fully. the SUnderland protocol is excellent, and also books by Natasha Campbell MacBride, Karen de Felize (A lot of interesting info on digestive enzymes), Marilyn Le Breton.
I would advise getting professional input, but have found nhs dieticians to be very dismissive of Sunderland et al - often without actually knowing what the protocol entiails. It seemed to me (and I have been through the system with both my dds) that they were caerign to the lowest common denominator - assuming I woudl just cut out some foods withut replacing nutrients...
agree with bochead on that - it is not abut cuttign out some food groups, it is about replacing them (if you go ahead) to ensure a properly balanced diet.
I have used a nutritionist with lots of success (form nutri-link), but have also seen a couple who were happy to take my money and give very generalised advice (ie seemed to know less than I did!). Ask aroudn if you want a private nutritionist - word of mouth can be a good place to start.
Take a look at Treating Autism, Talk Abut Curing Autism (hate the name, the site is good though) and Autim Medical (name might have changed - also might be subscription now) websites (sorry no links - am on phone)
I have done a gf/cf (plus a few other bits) diet with both my dds for years now. dd1 is severely ASD, dd2 is borderline AS. It was a fantastic thing for dd1 - she has really changed. Immense differences in her, and it remans the best thign we have ever done for her.
dd2 now had gluten, but it doesn't always sit well .she does, however, have a similar gut profile to dd1, and so I believe it was a Good Thing to keep her gf/cf from weaning.
as a starting point I would go via Sunderland: they have been involved in this area for years, and speak a lot of sense, medically/scientifically speaking. They are very nice and helpful too.