I've done just that
I've done just thatMate, for the price of €800 doesn't look like a good deal at all. If you're looking to restore it and have plenty of money to spend, maybe (if you don't mind the headaches of doing so).
But being a 125 and never seeing any maintenance AT ALL like that - and the non-stock parts put - I don't feel like even €350 is even fair.
I'd personally buy a bike that gave me some assurance it will work optimally for at least some time, given repairs I could afford mentally and financially.
But if you're really into it and can't stop thinking about how amazing this could be, then this is all bs, just go for it.