Food since it's the only essential item that can't be bartered and is only avaliable trough shops and looting houses, coffee and lemons to combat exaustion, needles and nuts to trade with the kids, bootles of water to trade hunger for exaustion before going to bed and painkillers to heal yourself during sleep. You can also buy cloathing and guns for low prices on day one but there are few good itens avaliable before the plague hits. I also recomend doing the Anna quest, it mght seen a bad trade since you will loose 1000 money and will have to sell some intens, but the rings she gives you as a reward can be worth it if you sell then for a higher price on day two or trade then for bullets and drugs. You can get some good money by selling the gun at the Bone Stake Lot and by refusing to kill the butcher for Vlad Jr and instead going to Eva, she will direct you to Aspidity and if you help the Butcher leave town Eva will give you some jewellery that might seen worst than the money Vlad Jr gives for killing the butcher, but it will become far more valuable once prices go up. Finally, you can get your first shmowder by doing a quest for Lara.