Anything additional we need to do as UK citizens living in South Korea?

Question

Myself and my girlfriend are both UK citizens who are currently living and working in South Korea. Starting in January we're hoping to travel through China, then take the trans-mongolian route through to Europe.

We've read that we'd be able to get a Russian visa over here as we are temporary Korean citizens. We're going to apply for the invitation through the Way to Russia website, we just wanted to double check there's nothing additional we should add with us currently living over here?

Also, how long is the invitation valid for? We were hoping to get that out of the way first, then sort Chinese and Mongolian visas before finally sorting the Russian one, as we won't enter Russia until early February.

Any help would be greatly appreciated!