Sounds like an idyllic part of the world, lucky you! I've heard really good results about the Ixworth, plus you'll be preserving a rare breed.
Sadly a lot of our traditional breeds have lost their true characteristics over the years due to modern farming techniques so they are not the same as what they used to be. When buying Sussex, buy with caution as some may be good layers but no meat on them. If you can get hold of them, the White Sussex is often the closest to true Sussex traits as it was rarely exhibited so wasn't really mucked around with. They are ultra rare so may be difficult to source.
I have little experience of RIR but I should image their current status is the same as the Sussex.
My only advice would be to put in the time and effort to get the most authentic strain of the breed even if it means travelling a bit or paying more in delivery if there isn't anything decent locally. Hope this helps, Robin