If you're going to use the Grenade Rifle/Launcher, then Demo Expert x3 is pretty much mandatory, and you'll probably want Splash Damage and Hit the Deck as well. You'll want a 7 STR before implant as the LMG has an 8 requirement, and a high AGI is probably not a bad idea although not really required. Note that those will not be obtainable early on, so the Grenade Rifle and 9mm SMG are good 'starter' equipment (there's a GR in Nipton in the booby-trapped house, and 9mm SMGs are all over the place).

My Infantryman went with the green Combat Armor as it's the most fitting, and his weapons are the LMG and Grenade Launcher. If you're going to be using only weapons covered by the Grunt perk, then Better Criticals will not be of much use to you as you won't really be getting any. Pass on Hand Loader, it only helps the 45s. Other good perks: Demo Expert X3 for awesome Grenade Rifle/Launcher, Hit the Dek, Commando/Gun Slinger if you like Vats. Tag: Guns, Explosives and what ever you want. A 7 strength is good, take 1 str implant and you can use the LMG which is a beast with AP/HP rounds. So you will not get much mileage out of them. All the weapons related to the Grunt perk has a low crit multiplier (except the Combat Knife, but it is not very impressive).

The full auto weapons will destroy anything that gets close - why pull out a tooth pick when you can pump all kinds of rounds downstream fast?! I would pass on Chances Knife/Combat Knife. First off, on your weapons list you are missing some true gems that work with the Grunt perk: Vances 9MM SMG, 45 SMG, Assault Carbine, Light Machine Gun and Grenade Rifle/Launcher.

Fallout new vegas grunt