Money was almost an issue for me until I realized I needed to do all of Tang's car theft jobs for 100% anyway. Some of the cars go for a few hundred thousand each, so you shouldn't be left short. This can also be mitigated by the Dai Lo perk in the Face upgrades tree and the Master Negotiator clothing set. I am not sure if they stack, but with just the 40% off that the Dai Lo perk gets you should be fine.
The actual execution of this is simply to go from one car shop to another, buying up everything as you go. There are two shops in North Point (chop shop and docks), one in Central (impound lot) and one in Kennedy Town (docks).
I would recommend waiting until after finishing most of the other content, as its a sluggish process that will clutter your un-sortable car list at your garage.
Edit: Enough people have commented that my cash value was off that I have removed it, because I'm not going to rent the game again to check my numbers again.