That all depends...       Not what you wanted to hear :)

.030 & .023 both work

Mostly it depends on what your trying to weld.    

The .030 is going to require a little more current than the .023  but totally doable.  

Avoid extension cords where you can.   If you need one, it must rated for the current you are pulling... A cheap extension will drink all your energy and blow breakers.

The gas will work, but the sweet spot in WFS vs voltage is going to be a bit tighter...  So it takes a little more tuning to get a spatter free weld... The up-side is that C02 will provide a little more penetration than 75/25

If it was me, I would use up that bottle rather than throwing it away.