Can it be done with a small bender like this one Sheet metel Bender
Or do you need a heavy duty type of bender.

No, you will need a special jig for bending the flats which basically consists of a top die that presses the flat into a bottom U-shaped jig that is the correct dimension.  There are prints available if you want to build one, or you can buy the jigs but they are expensive...personally I would buy a preformed receiver as they are inexpensive and ready to accept your parts kit.  Unless you are going to build a lot of guns the flats are not worth messing with.