A bit more than I think that bike would be worth in the US, but still looks like a nice complete bike. In my part of the US that is probably a $1,500 bike, so ~£1,200 or so.
Restoring to original would be expensive and you wouldn't get your money back. However, a brat conversion would be expensive too, plus more work, and you still wouldn't get your mone back.
I would do the minimum to get it on the road and MOT'd then ride it for a while to get a feel for it. But either way, I would caution you not to expect to get your money back. 550's just aren't as collectible as 750's and 400's.