I was also worried about placing the toilet in exactly the right spot. I was worried an unforeseen issue might arise it if wasn't precisely in the stock location. Now that I've done it I realize the exact location isn't really that critical.
The main thing to watch out for is the metal frame, as you thought. The drain hole is centered on the bottom of the tank. I don't know the exact measurements off hand and can't check right now. However, I believe the tank is about 5" away from the edge of the doorway- about 1" from the back wall. As for the sidewall, it curves so that is another story-- the ensolite actually comes off the wall and covers maybe 2-3" of the floor. The tank actually sits at least 1" over the ensolite. There would be more room to spare in that direction except the ensolite just beyond where I put it has LOTS of wiring under it, plus the copper gas line. The hole in the floor has about 1/4" clearance to the edge of the metal frame
Just buy the tank/toilet/drain plumbing/vent plumbing all at once from SCAMP
and go to town. Shipping is high from SCAMP
but will still be a better deal than buying from someone else. Getting all of the bits and parts for the toilet from Scamp
will save you a lot of head scratching.
Actually installing the toilet was very easy. Plumbing will be another story- I will be doing it all from scratch with just slightly more than zero plumbing experience.