From my own very limitted knowledge of NSR250s, the bodywork (if indeed it is original factory paint) would be from a 91/92 R model. The "GULL ARM" stickers did come on a few 91/92 models in place of the "250R" sticker - some of the 92/93 models had the "PROSPEC QUARTER" sticker. Check out this page on Jaime's site.
It's hard to tell if it is a genuine SP from just the photo's. But if he tells you that the bodywork was originally from the bike on sale, then it probably ain't a genuine SP champ... but it's still an NSR with some trick bits on there!
Bike's in average/poor condition always look better in photos than in the flesh and bikes in stunning mint condition look even better in real life than in photos and it's always hard to lay down cash on something you haven't seen or tested
I bought my MC21SP [Pentax Racing colours] from eBay too. I pretty much just took a gamble as the seller was uncontactable and I was desperate for an NSR250 (not a very common bike on the roads in Australia).
I hope I haven't made you more confused than before
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Jerry Lai
NSR250 [MC21SP]