Check the maintenance section for more info. Generally with any old cars, especially performance ones, get a full mechanical inspection done by someone who knows his stuff and you should be alright (e.g. leakdown and compression tested).
Everything you've asked largely depend on the person driving it. Some people flog it all the time whereas some other do it occasionally. To give you a rough idea, my gtr is pulling 250awkw and consume 7.5km/L or about 13L/100km. Thats with ocassional boost (not all the time but pretty frequent).
Maintenance is just about the same as any other car of the same age and as I've said before, you get what you pay for and getting it inspected is a top priority. That way you will sort of know what MAYbe failing and can budget for it or factor that into the purchase price.
Common problems are the clutch and turbo. Engine cost a bit to rebuild as well but if you get the engine tested, you shouldnt have much dramas with them.
If that $22k AUD is FOB price, you'll be looking at 1.8mil YEN mark. I find that really hard to believe a decent 34GTR will be sold at that price. Unless you're looking at something above 100,000km and maybe below grade 3 or 3.5. Anyway for that kinda FOB price you'll be looking at around $35k AUD on-road (varies a little with insurance and your local rego). A more realistic FOB price would be around 3mil YEN and thats more than $50k AUD.
Unless you can do all the repair or mechanical work yourself, I wouldnt suggest getting anything at the lower price band. You just dont know when it'll shit itself and chances are its pretty soon.