It is not worth buying the cheaper guns as they are no better than your starting pistols. It is better to instead save up your money and buy the more expensive, powerful looking guns.
Protecting Target
When you are asked to protect a target a good strategy is to just run back and forth past it. When you do this your hero will automatically kill anything that comes near it.
Reels
To make progress through the game you must successfully complete the reels. If you need to top up your total you can always return into earlier levels and try to master some of the objectives. Doing this will also earn you currency and XP.
Upgrade Weapons
Use the coin currency you loot from zombie corpses to upgrade any weapon you own by returning to the shop and selecting the 'Upgrade' option and increasing damage, ammo size and firing rates.