Okay, a couple of thoughts. You are correct that the Original Drake, CST743S (round 1.6) CST743E (round 1.28gpf) CST744S (elongated 1.6) and CST744E (elongated 1.28) only come in ADA height on the elongated bowl, not the round. The ADA Height original Drake is going to be model number CST744SL (elongated 1.6) or CST744EL (elongated 1.28). The bowl part is the same, the gallonage per flush just depends on the guts in the tank, so you have an E tank and an S tank for the different gallonages. The E works fine, and saves money. But you should feel comfortable just seeking the best deal.
And the Original Drake is a great value -- amazing flush, decent bowl cleaning, high quality porcelain, low defect rate.
There's also a new-style 1.28 tank available for a little more money. Here's a photo: CST744ELN is the model. "N" for "new tank".
http://www.totousa.com/ProductDetail.aspx?productid=1276 (Toto doesn't have a spec sheet for the ELN up on the web site; just the parts list.)
Unless you have a door that's going to get in the way, I'm here to tell you from experience that you should measure out where the bowl will protrude to and if you think it will fit and you can fit past it that you will be happier with that elongated bowl. I put a round in a small bathroom to save the space and I realized that I should have just put in the elongated, which I put in a different bathroom. You sit in the same spot; you just have the comfort of a little more toilet in front of you.
On the specs, the best way is to go to the totousa.com web site and look. The newest spec sheet will always be on that site; other vendors don't always (or ever) update the sheet they are displaying, so that's the best place to start. Here's the link to the current spec sheet on the ADA version: As you can see, it's 28" from front to back. Then there is 1.25 inches behind the tank when mounted on a (standard) 12" rough-in. So 29.25 inches from the finished wall (not the baseboard) if the centerline of the flange is exactly 12" from the finished wall (not the baseboard).
On your other questions, it does look like the Delta parts are available on the HD web site. The fill valve for the Corrente and the Prelude are both available online at the HD web site. The Prelude one is a Fluidmaster that you could probably replace with a Korky 528MP, the Corrente one is a unique one for $39. The flapper is also available online from HD; it's a unique size so you can't use a Korky.
I would pass on the Cadet 3; there's just no real reason to buy it and we still see complaints about quality control. Even recently, a guy who was buying AS toilets had to take one out because the rim holes hadn't properly been drilled/formed, which he didn't discover until he realized that it wasn't flushing right. I have never ever heard of that happening with a Toto.