I would guess that it weighs in the neighborhood of 25 pounds. That's a lot lighter than most new kids bikes made these days, which have overbuilt aluminum frames and heavy, mostly ineffective, suspension.
It doesn't look like anything would obviously need to be upgraded. The components are entry-level 90's components, probably Shimano, but that's still a bit better than what you'd find on a Walmart kids bike today. The brakes might need a tune-up from a knowledgeable bike shop (cantilever brakes are notoriously fussy), but if the seller is a skilled mechanic even that might not be necessary.