I've done just that . Got ot for 500, and so far, people that looked at it think it was probably worth it. But i'll try to upload a video for future reference, but also in order to upload on here. This way you and other members may help me assess problem zones and all. I'll pm you if you'd like. 👍🏾Mate, 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.