In a purely strategic sense, assuming the ultimate goal is to collect all the cars you're better off buying cars that aren't 20M first because they can sometimes go up quite a lot in price over time. 20M cars are at least going to never cost more than that...
what, the weekly race things? LOL...I stopped bothering with those ages ago. They almost always require at minimum an hour and aren't something I enjoy anyway which immediately makes running Sardegna twice for 1.5M the better option (especially when the big prizes include parts or engine tickets!). The only thing I'm really interested in doing in "game" terms is the weekly Time Trial events - which are also a money-loser in purely credits-per-hour terms but at least I enjoy the challenge (although the last few events have been more frustration than enjoyment). I really don't get much enjoyment from racing at all to be honest. I enjoy GT for the driving...all I really need are cars and tracks to drive them on.