I get by with a couple good small center drills, a couple good hard spotting drills...for number 4 screws a 33,43,42 for number 6 screws a 28, 36, 31 for number 8 screws a 19, 29, 28. That will 90% cover what you will need. Bigger sizes for guard screws, I buy as needed. One thing I like is a 3 flute hard drill in #33 for old milsurp receivers. IMO 2 flute taps are worth the money, I have never broken one (shouldn't have said that out loud).
