For your welder #6 is fine. In a residence 6/2NM is what I would use. You do NOT need 6/3 for this.
A two-pole 50 amp breaker is correct.
Is 30 amps the exact spec (running amps) for the compressor? If so then yes, #8 is good. Again, 8/2NM is what I would use on a 40 amp breaker.
At 35-40 feet voltage drop is not at all an issue.