Do not waste money on safe houses. You can make it just fine with the ones you get in the story. Instead, use your money for clothing and weapons. All the best weapons cost a lot but are well worth it. The Saturday Night Special is a better revolver and costs $75,000. The Assassin's Pistol is better gun and costs about $200,000. The Python is a better magnum and costs $250,000. The Dillinger is a machine gun and costs $450,000. Finally, the Street Sweeper, which works like a shotgun, costs $500,000 and will kill most people in one shot.
The pistol at level 3 works like a semi-automatic gun. It carries fourteen bullets and has more firepower. It is great for taking over businesses. The shotgun at level 3 has more ammunition, more firepower, and is great for taking over warehouses, etc. Prioritize weapons upgrades before safehouse buying. One house in each city is fine at the start. However, you will have to buy them all in the end.