Stockpot double as beer brewing pot
Brewer here. Yes, that should work fine.  

Material-wise, you want stainless steel so there is no leaching of metal when you start to boil, and no break down if you use chemicals in it later for cleaning (don't ever leave any chemicals in the pot though, follow directions, many chemicals will even eat away stainless steel if left too long). Either stainless steel or aluminum can handle the heat. Stainless steel encased aluminum pots actually tend to heat more evenly though (not much of a concern when brewing), so that's nice.

The only consideration you have is size.  40 qt. is 10 gallons.  Most intro batches of beer are 5 gallons. This would be perfect for a 5 gallon batch.  If you mash (as opposed to extract), you will collect about 6-7 gallons of wort (pre boil). This will boil off leaving you the amount you want.  See where I'm going?  If you up the brew from 5 to 10 gallons, the next most common homebrew size, the 10 gallon pot is not big enough.  There are tricks to get around this though, such as boiling less liquid and topping off with water to desired amount, but having less liquid affects hop utilization (not too big of a deal on a homebrew scale though). 

So, I'd say if you're just starting out, this pot size is fine. It will get you through intro size (5 gal) and will even work at 10 gal, depending on how you do it and what you want to do.  Anything bigger, you'll probably want pro and/or dedicated equipment.