it is very hard without boost to achieve such a goal i think. most straightforward, but hardest (impossible) way is hacking their servers. good luck with that
other way is playing the game. i never bought boost and only did speed boost offers and the maximum i could achieve was 4.6k i think? thankfully EA thanked us for putting up with all their BS by giving away unlimited cars 2 days ago :D, so I could get the points for 150 cars. but without that i dont think its possible. even if you buy every igc brand, win all the class races, finish all the parts achievements, and all the other igc possible achievements i believe you will still be a few hundred short of 5k.
so in short, you need to spend speedboost. even with all the hacks, of which some of the important ones are now patched, youll probably still be short =/