I'm not familiar with 1/0 or 2/0 wire. I guess that is just a thick
copper wire shielded. So I would need 3 of those to run Hot, Hot,
Neutral wire? Plus I would need a ground wire? How does that compare
price wise? Also, what is the best way to wrap them together, and do
you have to run it through condiut?

Also, the Millermatic 250, that I was trying to buy, fell through, so I
won't have anything that big now. I know I would never need anything
bigger than a Millermatic 210, but saw the 250 for a good price and
that fell through.

I just bought a Hobart Handler 180 reconditioned welder off Ebay today
for cheap, so will be using that for a long time. That is rated for 20
amps, and something like a Millermatic 210 or Hobart IronMan 210 are
rated at 25 - 27 amps. So now I know I won't ever need a 45 amp
welder, 25 - 27 amps max. Also the more I think about it, I will never
be using the Air Compressor and Welder at the same time. So I'm sure I
could take my Air Compressor off the list, math wise.

I think this may be good enough for the math

Welder, Tools Outlet, Outdoor Lights. What do these Welders draw
usually just being turned on and not used? Just incase I have it on,
and am running saws or grindings at the same time.

I'm starting to think now, I could easily run 6 ga wire and a 50-60 amp
breaker and be just fine. Any opinions on that? I can get 6 ga wire
pretty easy and cheap in my area. I can get that as low as $1.10 /
foot, where I got quoted $2.46 / foot for 4 gauge. I may still run the
4 ga though to be safe. Would cost me about $100 or so more to run,
but might save me in the future, if I don't need to up the amperage.