1) Middle sized frames are the most common, and often sell easily locally. The harder to sell bikes get peddled on non-local websites.
2) Foreign purchases will always be riskier, and more expensive due to both shipping and customs charges.
3) English road bikes are fairly uncommon, and a lot of the mid 70's Raleighs have odd thread pitches. This one might have an Italian bottom bracket, or an English 26tpi bracket. If you are looking to avoid non-standard threading and components, you would be wise to stick with Japanese brands.
4) Lots. Just do a search. Don't expect them to be inexpensive.
5) Tubulars rims are 622mmŲ. 700c rims are 622mmŲ.

Your knowledge of road bikes seems to be extremely limited. Are you sure you are ready to take on a project on a machine you know so little about? That bike you are looking at is worth maybe 1/2 of what they are asking, and the shipping is also overpriced. If I saw this bike locally, I'd offer $100, and maybe be willing to go up to $150. If you don't want tubulars, keep in mind that wheel building requires at least mid-level skills, which I doubt you are ready for. You should look for something more suitable-something that is more user-friendly AND is already set up the way you want it. You do NOT want a basket-case like this, even if you CAN get it for less than half of what this dreamer is asking.