Here is a easy way to obtain 100,000 plus credits fast. Get with a couple online friends. Setup a race for 30 laps, no matter the length(I did this on a 3.63 mile track). The winner and 2nd place will receive over 100,000 credits, 3rd place will receive around 50,000 to 100,000 credits. Races will probably last around 30 to 45 minutes but it's fun and worth it.
What to have some fun in R2, give the Mazda Furai a try. Not the fastest car on a long straight but it's great in the turns. The more turns the better. Go up on the tire size and passum in the corners. Go in deep, brake and go down 3-4 gears.
Ok so you don't want to go and lose a whole bunch of races in world tour so what you do is, go to rivals and look for races that have timeto beat about 15 and more minutes set to the harder dificulty and win and get about 9,000 plus a race or find one thats over 20 minuts to beat and set the hardest mode and stack cash fast
This car is already great , But don't give in to temptation of adding "power parts" , e.g exhaust pipes , turbo's . Instead install all availible handling parts , Racing tyres , Racing suspension and anti-roll bars , this will not only stiffen your car up but will lighten it and make it a hell of a lot quicker into and out of the corners.
To get the Speed Demon Achievement you have to reach 240mph, which is easier said than done!
The recommended technique for this is to use the Bugatti Veyron (though the SSC Ultimate Aero or Koenigsegg Agera also work well) on Le Mans Old Mulsanne Circuit. This has a very long straight, and if you play it in Free Play mode you can simply cruise through the first few corners and then put the hammer down on the straight. You should be able to reach 240mph with relative ease.
If you do not have it already, you can get the Flat Out achievement at this point, which seems to be awarded for holding a very fast speed for a good few seconds.