no one has answered the OPs question , what size of grinder does he need , ??
are u talking a 3 inch pipe or 2-3 foot diameter pipe ?
big heavy one imho… but im a girl , angle grinders are great things , will cut anything , but watch out cutting into heavy pipe if u get a big grinder , the kick back can be really mean ,dont cut deep into the pipe , keep moveing round and round the pipe
I’ve been cutting galvanized pipe for over 20 years with a portable electric 4" Milwaukee band saw. Works best when you are cutting outdoors and have lots of room. It’s too big to fit into confined areas. Then you would want to use a Variable Speed Reciprocating Saw with a metal cutting blade. Be sure to sweep up or shopvac your debris if you indoors or on a hard surface. If you have to cut a lot of heavy pipe don’t waste time with battery powered tools.
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It’s been six years. The OP has either succeeded, or given up by now.
Not sure if it helps in your situation, but you can put a sawzall blade in upside down and cut from the bottom up. If, for no other reason, to extend the life of the blade, but it’s usually helpful when you can’t access the top of the material.
One of my pet peeves is this. Safety glasses had side shields, I wear them every day at work. His don’t. He also wears a respirator with a full beard, he might as well wear nothing.
I had a job for many years where we made safety glasses. Not all safety glasses have side shields, and some that do have detachable side shields. Depends on the nature of the work. Also the notion that you can’t wear a respirator with a beard is a myth.
You have a Power Hack Saw(Reciprocating Saw) pictured in your head, However a “Band Saw” is not a “Back and Forth” saw 
In my last few months as a Fire Fighter i let my scant beard grow and didn’t plan on any interior work, however the best of plans can fall.
I doned a new MSA SCBA for a conveyor belt fire and opened the bypass valve for extra flow and could not believe how much smoke i ate! I had to bale out of the belt gallery immediately.
One of my Grandsons is an asbestos abatement contractor and his beard is 4+ time heaver than mine and he passes all the tests without a clean shave and large goatee.
I do worry for his health but he re-certifies his “Stos” and MSHA annually.
Get the Harbor Freight Model. It has a considerably larger throat and it’s a lot cheaper. I think I paid $80 for mine.
The plumbers I work with start out with an angle grinder and finish with a Milwaukee Diablo carbide blade in the sawzall.
Or the pipe snapper if they have cash for one.