72 hours is a good goal, and one even us low income people can attain, with smart buying. Solar chargers can be had for cell phones and flashlight batteries, and with the "add-one'can'per'trip" shopping, it doesn't take long to establish a supply in the home. Our parents used to have much more then 72 hours worth of food, but that called a "pantry", not "prepping".
Water, blue jugs from WalMart hold 6 gallons each with no issue, rotate your water by using it, and refilling these Reliant stack-on jugs, and they don't really take up that much room.
BIG rule...unless your neighbors are part of your plan, no matter if it's 72 hours or OMG ITS THE END OF THE WORLD, don't show off/tell 'em what you have. Some people "prep" with just guns and ammo, so they can just take your stuff. You won't need it when you're dead, right?
Looters are a fact of the human condition, whether the break down is short or long.
back to the point - start with what you think 72 hours might need, and yes, medical kits, (not your $5 WalMart First Aid Kit), and some First Aid training would be an excellent idea.