Instead of running around and grabbing all the cars, just sort the available cars by driveability and only buy ones that are running. Buy a few (later you can buy a lot at once), then return to the office.

Go to the computer and 'reset' anything more than 60 meters from the office. The vehicle will appear in the front of the lot. You can do this every two minutes. By just running to the computer every couple of minutes you will have everything you've bought on your lot without towing a single thing.

By only buying running vehicles, you do not need to tow/dolly them into the repair bay. This makes the whole process much faster.

What I do is go out at night and buy a bunch, then reset one into the lot, put it in line for work or start on it, then reset another, then repeat. No tow truck needed, no running around hooking and dropping, etc.