The Maximum Street Cred you can achieve is Level 50. This can be done throughout the game as you progress with Main/Side missions, NCPD and Gig Quests.
There is currently a glitch in the game where you can easily reach Rank 50 in the mission "I Walk The Line" - I got this mission around 40 hours into my game (could be less, as I've been going around completing most side missions first).
I was around Street Cred level 35 prior to doing this method and took me just under an hour to. In doing this method you can either get 15K or 30K Eddies which can help you get enough money to buy all the vehicles in the game (which is around 1.8 MIL)
I suggest doing this method before it gets patched.
The video guide below shows the best way to do this. Full credit goes to them.