Is this original? SM 2577 351

That’s a redial, I’m afraid and imo would be too expensive even if it wasn’t. Note the missing Swiss Made and incorrect script of OMEGA.

Too add a few details: It’s also been polished, relumed, the crown is the correct style but a later replacement without the flat foot logo and the clasp is a modern style, I’d have my doubts if it’s actually genuine. These things would bother me in decreasing order, though, with the refinished dial being the most impactful alteration.

This is hard to say, as it depends on your own preferences, really. I’d say to most people here it’s not worth much, as the collector value is gone due to the redone dial. Personally, I wouldn’t buy this for a quarter of the price.

In a no reserve eBay auction, I’d still assume this would fetch ~400-450€ or so.

If it was all original, dial and case in good shape you’d have a nice watch in a relatively common configuration and ~600-650€ wouldn’t be unrealistic, I think. That is if buying from a private seller - a dealer would likely ask for more.