Upgrading all cars as you go through the single player can cause problems.
You can earn enough cash in SP to buy all cars, but not upgrade them all.
You don't need to buy them of them to upgrade them all. Some are already class A. However, to upgrade them all, you either need to go online or complete challenges from friends.
To get online, you need the VIP pass, which was given free as GWG. I am right in assuming that you missed the free period?
The dashboard trick is where you have enough money for all the vehicles needed for the 1st achievement, but not enough to buy them all and upgrades. However, you may have enough for all requited upgradable cars plus upgrades (this is not the same list as all cars, see Spilner's solution), if you follow me.
As far as I know, a VIP pass is not needed for challenges and silver accounts work for the challenges. If challenges can be sent with a silver account, a VIP pass is no needed. No mention of gold membership or VIP pass needed in the solutions for
.
If you need any further help, don't hesitate to ask. I will do my best.