Starting dont spend money on unnecessary stuff like properties, cars etc. Just use the money mainly for weapons.
The only way to guarantee earn more than a billion bucks without whatever hassle is through lester's assassination missions combined with the stock market.
The infinite money thing dont bother la lol 1 time get at most 25k, you do few hours also not more than 4-5 million per character.
After finishing the storyline just follow the guide, within 4-5 missions can earn more than 1 billion per character. After that can buy whatever and do whatever u want liao.
How To Make Up To 1.5 Billion With Lester's Assassination Missions : GrandTheftAutoV