So here's my recipe. While the advice of grinding seasonals for credits is definitely something you'll need to do, there are some other things you can do to get there more quickly.
1) As mentioned, build up a war chest of around 10,000,000 credits. You'll need this much because the more expensive cars are not tradeable, so you are going to have to buy them or win them.
2) Check this thread
https://www.gtplanet.net/forum/showthread.php?t=168769 and download
The Matrix. This will give you a list of the best payouts on the seasonals and provide you with a nifty way of keeping track of your cars, gear, horns, and museum cards.
3)Buying will get you so far, but if you want to maximize your collecting power, you'll need to trade. Check this forum
https://www.gtplanet.net/forum/forumdisplay.php?f=239 for folks who are trading, what they are trading, and what they want in return.
4) What to trade and to whom? The what is whatever will get you what you need. In general, the most valuable cars to traders are 0/0s that can be won or obtained from the OCD. I encourage you to start a second account and do all the of license tests and easy A-Spec races to get as many of these type of cars as possible. Also, when the OCD switches, buy up everything that has an actual color chip. Many people either collect multiple colors of the same car or they want the chips. This is a great way to get cars you need. In addition, if you are trying for cars first, sacrifice everything else. Trade gear, paint, MCs, and horns to anyone willing to trade those items for cars. The who is more complicated, but you can start with this thread
https://www.gtplanet.net/forum/showthread.php?t=136295. It isn't maintained anymore, but people still post feedback there that is invaluable in determining who is a safe trade partner.
4a) Once you get trading, check the sigs of the folks who trade a lot and use their scheduling documents as a template to make your own. The one thing that will kill you in trading is if you do not keep your deals. Having a schedule within a thread or on Google Docs will help you keep everything straight so you don't miss trades and lose reputation.
5)Visit this thread
https://www.gtplanet.net/forum/showthread.php?t=134403 and get involved with this project. Take the time to learn how it works. If you are lucky enough to be in a known part of the database, you will always know what cars are coming in your UCD and be able to minimize the time it takes to get them all. Famine's initial work has been taken over by others, so stick to the end part of the thread. Segovia and TornadoAlley are invaluable resources in learning about and using the database.
6) Find a benefactor. There are a handful of guys out there that are wonderfully generous and seem to take getting everyone to 100% as their job. I won't mention names, but you'll find them if your trading (or more likely, they'll find you.)
I think that's it. I didn't say anything about DLC, because a good part of the original DLC content is not available and untradeable.
