Machaggis, welcome to my world
This could either be the most entertaining project you have ever undertaken or a slow progression of Dante's levels as you descend into...line engraved stamps. Just this week a stockbook of Japanese stamps arrived and most are photo not line engraved so slim pickings there.
I have yet to find a 'definitive' guide but if you find one please do let me know as it will stop me wondering aimlessly through worldwide.
So far I have decided that Geo V, VI, Edward and the early QEII sets all qualify from the commonwealth but that's just the start. I have decided to exclude the early 'heads' as these are very expensive and to my eye not as fabulous as the later line engraved and intaglio issues (and I've tried to stick to intaglio but got sidetracked). On top of that I've dived into Czechoslovakia, Sweden, France, French Colonies, Belgian Congo, some Belgium, Finland, Denmark and the odd Italy and Monaco. Then there's Central and South America where there are literally thousands of line engraved and intaglio (Nicaragua, Peru, Mexico....the list goes on) and then the other bits of that Continent so USA (thousands) and Canada (up to 1966) plus Newfoundland.
And all this before we consider the Asian market of China, Japan, Hong Kong, Laos, Cambodia, Burma.....Australia of course and New Zealand plus PNG, Norfolk Island, Christmas Island.....the list goes on.
So I've given up and am simply adding on anything that looks nice

It means that often I end up buying sets because they look nice but then finding that I'm missing the top one or two values, so that's another job to find them and fill the space, as I hate incomplete sets. Not overly worried about them but I do like the completeness you understand.
Part of me thinks I should just stick to the intaglio stamps from Part 1 but then I miss all the wonderful French and French Colonies and so much else from around the world. I've discovered the only way to do this is to buy one of each Part SG catalogue as this is the only way of seeing definitively if they are line engraved, intaglio or photo/litho masquerading....
I would welcome any further thoughts on this as I need a way of structuring this better! So far I have a dozen ring binders and thousands of stamps none of which are worth much but great fun to look at!
Jon