Barnet marine seconds are normally in reasonable condition, with just some scratches etc, you might not need to paint it depending on the colour you want, it should not have cracks I don't think. But, could have will have defects, scratches, chips, blemishes or streaks in the fibreglass as per the ad.
A good paint job would hide most of that with any chips filled and faired.
The bottom edge needs trimming I think, because it is as it came out of the mould.
There is no flange, so will have to be glassed to the deck, there's no way to screw it unless they can also sell you a flange to glass to the seat pod first.
You could try here to save all that work, buy the stainless backrest, and have the cushions made from scratch:-
I have not seen them to be able to comment on the quality, but have met Richard a few times, and he's a nice guy. They have a flange too, so no need to glass to the deck. Less work than the whole fill, fair and repaint option for maybe only £100 more