Yes although property is good and very reliable doesn't mean you shoudn't buy things from the inventory. No i don't mean to buy a weapon or car you think is cool, but buy things that allow you to do high payout jobs. This is what I do and make billions. :)
You also should make sure that once you are able to rob, you need to purchase enough armor equal to your level, and you need just as many mafia. Otherwise you will be robbed constantly and lose your income. It is expensive to fix and protect your property.