Ok as I recently got this set up I will chime in.
1. In order to force carb a keg it is still reccomended that you use straight CO2. You can however naturaly carb in the keg, or you can use the Beer Gas tank it will just take awhile. So realistically you need both a CO2 tank and a Beer Gas tank.
2. You would be asking for and using Beer Gas. Not Nitro, N2O, NO2, or straight N. Beer gas is a mix of N and CO2, the mix I have is 70% CO2 and 30% N. So your beer is still carbed with CO2, the N just makes it so you can serve at a higher PSI. Beer Gas at 30PSI will carbonate like straigth CO2 at 10 PSI.
3. There are two reasons to use Beer Gas. The first is you have really long lines and need to use a high pressure to deliver your beer and not over carbonate. The second is you want to use a stout faucet that has a restrictor plate. This plate whippes the beer into that creamy head and knockes most of the carbonation out. The origional intent is to replicate beer served through a Beer Engine with a sparkeler, but not have to deal with cask/real ales.
4. I use a regulator with an adaptor for my Beer Gas and use the same for my CO2. However my regulator is rated for CO2, N, and Air. The high pressure side needs to go up to at least 2000 PSI.
So if you are looking to get a Beer Gas set up the cheapest way I have found and what I have done is the following:
Kegorator
Stout Faucet with removable restritor plate.
CO2 Tank
Beer Gas Tank
Regulator that can handle both CO2 and Beer Gas
Adapter for CO2 to Beer Gas.
I am able to serve beer either on Beer Gas or CO2 this way.
Where I exchange they will take a CO2 tank and give you a Beer Gas tank. Sometimes I recive a CO2 tank that has been fitted with a different adapter changing it into a Beer Gas fitting. I then take their adapter off and use my CO2 regulator with out needing my Beer Gas adapter. Another thing to note is that at the place I exchange CO2 cost $10 and Beer Gas cost $24 both for 5 LB tanks. The Beer Gas will also not last for nearly as many kegs, because you are using a higher PSI and not as much product can be put in the same size tank.