good choice in going for a classic
re your post, you may not need to get it converted from km to mph - i've got a little paper conversion chart wedged in my dashboard and that does the job. You'll get to a point where you'll memorise the km speeds anyway
map for UK fuel - not an issue, you'll be wanting a remap anyway to get the best from the engine and/or any mods you might make
would recommend the import route if you are not in a rush, chapman seems to find great cars on auction and its worth the hassle, as there is a lot more choice over in Japan.
the registration process once it is off the boat is ok and tbh the importer does all that stuff for you.
if the Type R isnt exactly what you are after would recommend a RA - a great model of Impreza and comes with a lot of goodies as standard