Specification wise, the 700/750 800/850 are all *mostly* the same. The 7xx has cold cathode for lighting, whereas the 8xx has LED, so there's less warmup time. The 8xx series has different holder compared to the 7xx series-- in general, they hold fewer negatives, and have ANR "glass" on them. They also have more height adjustment steps. The 8xx series medium format holder is a bit of a pain because the tabs that hold the ANR material in place make it difficult to get the 120 film "just right" in the holder. But it does a better job of holding it flat.
Rumor has it that the 750 and 850 have slightly better coatings on the lens-- but I haven't found anyone who can confirm that.
Note that all four scanners have two scanning modes-- a "high resolution" lens (6400 PPI) that is supposed to be focused +3mm from the glass, and a "low resolution" lens (4800 PPI) that is focused on the glass bed. I forget the steps on the 8xx series holders, but I think they're about 0.2mm-- and yes, that much delta in height does make a difference in sharpness.
You won't get 6400 or 4800, but cut those numbers in half, and you'll probably get pretty close. I find that some sharpening always benefits. Personally, I prefer a technique called "Frequency Separation" that allows finer control over whether you're sharpening the overall image, or just the details.
To the OP, if the seller hasn't shipped yet, insist that they ensure BOTH transport locks are engaged.