The two most convenient web sources for rigging line are Model Expo ( www.modelexpo-online.com ) and Bluejacket ( www.bluejacketinc.com ). Bluejacket carries cotton (not recommended), nylon (ok for the finest lines, but not very good for bigger ones), and linen (great stuff, but it would have to be dyed - and expensive in the necessary quantities). Model Expo, under the Model Shipways label, sells that nice "cotton-poly mix" that - on the basis of limited experience with it - I rather like. I particularly like the "hemp" color and the texture of it; it really does look like rope.
The best (and most expensive) model rope available probably is the material called "Admiralty Brand" linen. It's quite expensive, and the firm that sells it is currently evading my senile memory. I'm inclined to recommend the Model Shipways stuff.
Bluejacket and Model Expo are also good sources for some fittings that you're probably going to want to buy eventually. The blocks and deadeyes that come with the kit really need to be discarded. Styrene plastic is a wonderful, versatile material, but there are some parts to which it just isn't suited. A rigid mold cannot produce a block or deadeye with holes through it
and a groove around it. The Heller designers came up with some clever dodges to get around the problem, but they don't really work.
Again, this is a problem that's a long way in the future, but you'll probably want to buy replacement deadeyes and blocks. Some modelers like the wood ones available from Model Expo; my own personal favorites are Bluejacket's cast britannia metal blocks and deadeyes. They aren't cheap, but the expense can, if you like, be spread out over a long period. (You only have to order a dozen at a time.) Yesterday I included links to some pictures of my models; they're rigged with Bluejacket blocks and deadeyes. If you play around a little with that site ( www.drydockmodels.com ) you'll encounter plenty of pictures of models rigged with wood blocks and deadeyes. Take your pick - but the plastic ones in the kit are just about useless.
The Longridge and McKay books probably contain enough info to establish the diameters of line you'll need. The best source on such topics, though, is
The Masting and Rigging of British Warships, by James Lees. The diagrams and verbal descriptions in that book are, I think, about the clearest description of the topic in print. Another book that belongs in your library is
Seamanship in the Age of Sail, by John Harland. It doesn't contain as much precise data as the others, but it's a wonderful treatise on how old sailing ships actually worked. And the illustrations are spectacular.
Hope this helps a little. Good luck.