My reinterpretation of this map is that the Doomguy entered a house to look for supplies, and then, the hellspawn ambushed him. You SHOULDN'T go with all guns blazing since the beggining. First you must get upstairs and get the chaingun and the shotgun. Jump from the window and yet don't engage any enemy. There are many options to get started: You can either go west and find the super shotgun in the pub, then get into the other house to get the key to open the gunshop, or right after jumping from the window, go to the house to the north to get the blue key and have some advantage by forcing the monsters to all come from the same place to get you.
I might still put more ammo in the starting, but remember that this is an error and trial map. I personaly finished the map twice with vanilla weapons, and Brutal Doom, and it was very easy.