Well here is what I learned as a long time Snapper owner..
The only "iron bore" Briggs engine made is the Pro 850.
A "Pro 10.00" has iron too but are not found on Snappers or any lawn mowers I've seen or heard of but could be? It's found more on log splitter type equipment.
Neither Pro 8.75 nor Pro 9.00 have an iron sleeve cylinder, just standard aluminum bore engines with Pro stickers.
There are 3-4 Snapper deck designs today.
Best Models: The old rear disc drive units with a drive disc on top of the deck just behind the engine. Parts are cheap..Real Commercial models have extra handle to deck braces and steel ball bearing wheels.
Hi-Vac & Ninja decks are indeed different, so please read Snapper website and get a model number from there "First" as details/pics on these mowers are all wrong here. Go visit Snapper then buy model number here..Double check numbers!!!
Worst Models: Any Snapper made with a plastic front end regardless its Style/Stickers/Name It is not a real Snapper it is in fact an MTD built lawnmower and will be absolutely junk from the minute you started it.
Most have very low grade engines Briggs makes today.. They are quite fragile in all aspects and only built to last a few seasons (Throwaways) unlike a REAL REAR DISC DRIVE Snapper deck that will last you decades as long as severe rust don't impact it. Keep its deck covered and dry it will outlast several engines and you. I have 3, the newest is a 1994, it's deck is still perfect so I just installed a new Briggs Pro 850 on it.
My advice is buy a rear disc drive Snapper Commercial with steel wheels regardless Ninja or Hi-Vac deck design choice, even though all Snappers are 3n1 mowers Ninja's mulch much better than Hi-Vac, Hi-Vacs bag better Ninja, buy what you desire.
Hope it helps...