Yeah you can run 5 heat range, I just find it easier to grab 6 range as they're everywhere (and I buy two sets, for both my cars). The car is 15+ years old now, even though he isn't having ignition issues, I don't see why he can't run 0.8mm gap. Wouldn't make much difference between 1.1 and 0.8 on N/A anyway. You don't need to gap down the 0.8mm version since it's already made with that gap. OP can try both gaps and heat ranges to see if it makes and difference. That's why I recommend getting the copper ones, $3-4 vs $15-20 per plug.
I've been running BCPR6ES on my R31 for 5+ years without issues. Ran both gaps and didn't make a difference.
Also, servicing the car isn't any different than servicing a non-Skyline. Oil + filters. Do the spark plugs and air + fuel filters if they haven't been done this year.
Oil filter - use Nissan (don't know part number) or Z145a.
Oil - whatever you like, but something like 10W-40 is fine (support Aussie brands, Nulon or Penrite).
Apart from the consumables, you'll need an oil pan/tray, 14mm spanner, filter opener thingy, and optionally a funnel and jack. Oh and rags, they come in handy. Remember to do up the sump plug before filling it with new oil.