I've been wanting to take the time to come back and answer the questions from this thread. I love the diversity of responses! Thank you all!

how long was the really really hard times with DS1 and DS2 together? Do you think that it could last as long again?... in our situation DH and I tend to want to prepare for the worst
You know, a big part of my H's problem is that both our children have been REALLY bad sleepers and I think we have adrenal fatigue as a result and my H is scared of not being able to go through it again. I feel like their sleep was largely due to their digestive troubles though and we could have at least a slightly improved sleeper 3rd time around. We're thinking we need to be prepared for the worst though too. That puts me off a bit

The reason I asked about the school is that I'm wondering if you will be put into a situation where you need to pay for some sort of private schooling.
My belief is that asd is genetic and that the cycle cannot be stopped. I'm guessing that's different from your belief?
We're fortunate my H is on a high income so we can factor in private school if need be.

I believe that even if ASD is genetic (a gene hasn't been found at this point) it's expression can be affected by your health and biochemistry, as in epigenetics. We have so far seen our son improve from severe-moderate autism to moderate-high functioning autism so we have witnessed first hand the effect health and nutrition has.

How old are your kids now Meow?
They're 5.5 and almost 3.

I think if you really want this, you'll make it work but if you can be swayed by justification against (like we were) then the desire probably wasn't that strong to begin with (we never had those feelings that someone was missing that others often explain for example), we had lots of reasons outside special needs that influenced our decision also. Is the special needs concerns your only big hesitation?
This started to occur to me too. Every family is different and just because we may have some of the same reasons for having a certain number of children doesn't mean that those reasons will add up to the same decision for everyone. Special needs isn't the only consideration. Sleep and the first two years scares us mightily! But special needs is a big consideration. I just had a terrifying experience trying to get the kids into the car parked on the street yesterday. It was hell with two children. With three I don't know what would have happened... but I've gotten used to usually planning around avoiding those experiences so I guess I would just keep doing the same. And hopefully DS2 can be more trusted soon.