Personally speaking, I would mention it. If a school is going to be negative about the disability, best you find out beforehand rather than afterwards, maybe? I'm not 100% sure that you are obliged to mention it, no, but if you didn't, and then he didn't cope, it could be awkward?
As for whether he'd be differently placed with or without a Statement, my understanding is that legally schools must not discriminate against a child for a disability. But whether that means they don't is another matter. Like they said on Pirates of the Caribbean, "...there's only two rules - that what a man can do, and that what he can't". A school that wishes to be naughty will often find a way to do so.
The only legal reason they can give for not accepting him is that it would cost way way too much or that he would be a danger or other huge problem to the others in their schooling. In which case the local education lot have to think hard about where to place him, and whether it is going to cause him big problems to go somewhere else.
A friendly head teacher and SENCO are a blessing, I found. An unfriendly set, a nightmare.
As for what to ask, I asked the school what it could do for ds, what provisions it had, what would happen if he didn't cope with the work etc. I saw the extra facilities and met with the team that would do the support. They've mostly been great. Worth asking to see children of his age and situation being taught, if they'll permit it.
Also, you'll have to 'think ASD'. If you possibly can, close your eyes in the classrooms or hallways for a moment and just listen. Whatever you can hear may be 10 times louder for a child with an ASD. See if you can see lots of movement and jostling and chaos in the classrooms - those will be a real challenge for any ASD child. Ask about a quiet zone, somewhere safe for an ASD child to go if they need it, and what sort of things they'd plan for break times and lunch times (generally we find socialising really hard and so breaks are harder work for many of us than the lessons, and we end up stressed out and exhausted).
Umm, all I can think of right now. Sorry.