can't find a massive amount of feedback online with regards to performance and things to look out for.
Not unusual. Boats with smaller numbers built and lesser known designs are not as prominent on the web as production boats. Your Colvic Countess 28, when compared to the Hunter, will have more interior space. Be a bit slower and less reactive to the waves and choppy seas of the Channel. A smoother ride. You will sit on the mud when the tide goes out, not having to raise the center board or rocking on the rounded hull.
Go face to face in the marina with boats of similar design and talk to the owners. You’ll learn all sorts of interesting things.
Things to watch out for are likely the same on all boats. Solid hull, no soft spots on the deck, rudder that rotates easily without wobble, standing rigging that is free of rust, an orderly appearance to the boat, no leaks, tankage enough to meet your needs, etc.
She will be a good coastal cruising boat. Bit small and uncomfortable to take far from shore.