I just added up an F-16C for fun, it's one of the cheapest fighters in the game.
About 19 million will get you to lvl 17.
It takes about 40 million to get to lvl 20.
So no, you cannot get a good plane to lvl 20 with 20 million credits. It will only get you halfway there with an inexpensive plane and somewhere around lvl 15 (or less) with more expensive planes.
As far as if it's worth it to shoot for lvl 20 with your ADF-01, I tend to agree with TLMguy. There's probably not much point going past lvl 15 other than posterity. Even if you could earn 5,000 more credits per mission with a lvl 20 plane (which is not likely IMO), it would take over 15,000 missions before you were seeing returns on the 60+ million (or more, there is no data on the price chart beyond lvl 16) it would take to upgrade it that far.
That being said, do what you want. I have 4 main planes, 2 are at lvl 15, the other two are close. At this moment, I have no interest is starting over with another plane just for the sake of variety, so in my case, I'll probably just keep dumping credits into the planes I use until I get completely tired of or bored with the game.