First, congratulations on selecting the Hi-Power, a classic design by John Browning and one of the most prolific firearms used/issued in the world. I have a small collection of FN Hi-Powers, and FM variants, the Arcus, Inglis and more recently the L1A from Indian Arms Board.
As far as how much lubrication on the internal surfaces, what I do is after using brake cleaner to blast out the big chunks, I go over the internal surfaces with a shaving brush that has been lightly (very lightly, like one drop of Castrol gun oil, but pick your own) oiled. The rails get one drop each before the slide gets fitted back on the frame. Too much lube on the internals will actually form a 'lapping compound' made up of the oil and gunpowder residue.
You might also consider a quality holster (I like the models from Kramer in horsehide), the range I visit gets a bit antsy if someone takes a firearm to the shooting booth that is not in a holster or zipper case. Before I forget, a pack of 'dummy rounds' will allow you to practice your trigger pull in the comfort of your living room (burnishing the trigger/sear/hammer surfaces in the process). Consider it a cheap mans trigger job. Extra magazines are both plentiful and inexpensive.
I have removed the factory grips and replaced them with a set of thin cocobolo grips from Spegel. The sights are pretty rudimentary, and a set of tritium illuminated sights will help when things get dim (in addition to being able to find the pistol). The recoil spring on my personal Hi-Power has been replaced by a unit from EFKFireDragon.com. It's like a small internal shock absorber to help manage the recoil (you can't get away from recoil, but you can manage it), drops right in, and requires no machining. Something to consider.