Embr

You are going to need a lot of in-game money, coins and diamonds to buy every vehicle cosmetic, tool and upgrade. There is no need to buy all gear.

Earn Money Quickly

A fast method to earn money is to complete the Salvage job in the level "Crab‌ ‌and‌ ‌Go:‌ ‌Claw‌ ‌Club‌" from the third district. This job lets you earn over $2,100 in 30 seconds.

This method is shown off in the YouTube video "Embr - Fast Money Grinding Method" by Nick Nelson:

Earn Coins and Diamonds Quickly

A fast method to earn coins and diamonds is to steal bonus cash with coins or diamonds, and then abandon and restart the job. You will still earn coins and diamonds for failed jobs. Bonus cash with coins or diamonds is guaranteed to respawn after restarting a failed job.

This method described in the Steam Community guide "How to get diamonds quickly" by philip and Crankodile:

After finding the diamond cash put it into your car and now DON’T finish the mission and instead abandon it. This will make you fail the mission and give you zero pay out, but it will reward you with the diamond you collected. The great thing about this is that if you start the job again you will encounter another diamond money. As long as you don’t finish the mission you will keep finding a diamond cash every round. Important to mention is that it will not always be the same stack of money that turns into a diamond variant. So you need to check all the cash in the house every round.

I recommend doing this at a house where you exactly know all the money locations and can get there quickly. It woks even better with a friend since you can split up. Me and my friend did it on the first house and managed to take an average of 15-20 sec per round.