Main Gun: Kroot Rifle. It has range, it has stopping power and it has the added bonus that should a zombie sneak up on me, I don't need to waste time taking out my melee weapon. Failing that, a Storm Bolter.
Side Arm: A Stub Gun with Dum-Dum rounds. Again it's got stopping power, and the ammo is damn well everywhere. 2nd choice would be like everyone else has pointed out, a laspistol, for the simple fact that you can charge the packs by leaving them in the sun or just chucking them in a fire.
Melee Weapon: Scout Combat Blade. It's reliable and the size of a sword, plus it's mono-molecular edge is going to give me the upper hand against unarmoured enemies, which zombies tend to be. Makes it easier to slice off heads and limbs.
None of these weapons will really slow me down in any real way, well the storm bolter might if I take it, but I can holster/sheath/sling the rest and still be able move with ease. Also, they're a fairly available choice (so long as there were some Kroot stopping by your world). My first choices are also solid slug ammo, so would be quite easy to make it myself. The same goes for the survivors in the main group listed later, all solid slug, easy maintanance weapons, such as autoguns, shotguns or las-weapons.
Vehicles: I was leaning towards some Dark Eldar Raiders, but being open you wouldn't be secure when sleeping, and supplies would just slide off unless tied down, and you'd have to stop and land to get the supplies between vehicles.
I eventually decided on 2 Ork Trukks (modified of course: enclose all the compartments, boarding planks to transfer people and supplies between the 2 Trukks while moving, and a nice big dozer blade at the front to clear away any zombies.).
Add to this 3 Buggies, 1 with Rokkits, to run as outriders and long range scouts.
And finally Battlewagon with plenty of Big Shootas to act as a mobile strongpoint to house most of the group's survivors and supplies, with everybody inside also armed, it'll make the zombie's life hell trying to storm it. The Trukks and Buggies go out foraging, and bring the majority of what they find back to the wagon for safe keeping, holding onto only what they need to keep going for another day.
Foraging trips would be made in pairs, 1 Trukk and 1 Buggy in both groups, with the 3rd "Rokkit" Buggie staying with the Wagon. The passengers in the Trukks disembark and quickly seach the area, with the Buggy patrolling the area, and the Trukk with an open view of the area. Both can then provide cover fire to the team on the ground if they run into zombie trouble, giving them time to get whatever supplies they found back to the Trukk. If surrounded, the Trukk takes the lead and plows through the zombies, with the Buggy following in it's wake.
I'm considering dropping the Rokkit Buggy and taking a second identical Battlewagon, that way we can carry more supplies, and at night the Trukks and Buggies can park between them and shelter from the zombie hordes under the sweeping seachlights and big guns of the Wagons.
How long are we stuck in the middle of this zombie apocalypse for anyway? Do we have to reach a certain place to be saved, last a certain amount of time for relief from the PDF? Or are we merely surviving? Because that's what I've gone for, slowly moving across the land, taking any supplies I can find as I go, and just holding the group together for survival in general.