oil system, what to buy??

Crank Scraper- Not sure on a 4 inch crank. May have to modify one for a stock crank. Either way not really worth the hassle IMO.
Spring- I would use the one supplied with the pump.
Priming the pump- It wouldn't hurt to pour some oil in the pump before installing it, that way it is not dry. Some people even use vaseline in the oil pump, not that there is anything wrong with that. When you prime the pump the oil pressure will come up and it will slow the drill down quite a bit. It helps to have a real good drill, I just burned up a cheap electric one on my race car this spring. Make sure you turn the motor by hand until you get oil to the rocker gear.
Oil Pressure Gauge-Doesn't really matter as long as you have a gauge. Just run a T out of the oil pressure port and you can run the stock sending unit and an aftermarket gauge.