If you've owned early Scorpion and Ultra MMCs then it's pretty much nothing new aside from a regulator and the lovely fore end on the stock.
The regs need servicing or replacement as will fail. If this is a private sale, definitely chrono it before paying! Very few people will take on a Hornet reg for service or rebuild, and not sure on which aftermarket regs are available these days, but probably a Huma one.
Cocking bolt can get graunchy if neglected, but simple to clean and re-lube.
Looks like very thin bluing on that reservoir, so check condition of that too. These are all probably 20+ years old now, so don't just bang 230 bar in it. End cap also looks incorrect for the model, but could be wrong.
Personally think that massive mod isn't in keeping with the rifle, but horses and courses! Also not sure if that is the rifle length or carbine from the pics, and carbine can limit type/size of mod used for end cap clearance, from what I remember of mine.
The LOP to the MMC button is not as natural as the Scorpion or Ultra, and can dictate your hold, but no big issue, depending on what / where you're shooting.
I'd maybe ask your unrelated Optics questions on a thread in the optics section else you might not get responses and risk jumbling up this thread.
Loads of threads on here about the Hornet, so try the search bar and you'll soon find loads of info.
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