Always remember the outfitting cost when purchasing a new hull. Budget a good 100% more. We all try to 'cheap out' when stepping up, but your poorly outfitted new ship is a miserable experience compared to what you were dreaming of. And the first time you get pulled out of SC, with minimal shields, no SCB's, no boost because of the D class PP/PD, and no re-buy... You'll be joining that club nobody wanted into, no-one admits too, and leaves a mark.
On your budget you have a few options. But you have to decide on which route you want to pursue. For 20 million credits :
Trading : T-6. Carries enough to get you a decent profit in bulk trading. For Rares, stick with the Cobra or step up to the Asp-X and the much better jump range. Ignore the T-7, also known as the 'flying coffin'. Trading has always been the way to reliably quick money.
Exploration : Asp-X.. jump range rules in getting you 'out there' and to some of sparse areas ( top, bottom, or between the arms ). Exploration seems to pay better then it once did. Go bounce around the neutron fields for a week and you'll come back rich.
Combat : The Cobra works here, but I'd swap to a Vulture. Great ship, and cheap fully A-fitted ( 20-25 million ). It's a ferocious little killer, and the best small dedicated combat ship. Engineering is a must, tho, since in stock form it has power issues. An Asp-X can work here. Never been a fan of the Viper series, but I will admit they are tough little mongrels.
I'd suggest the Asp-X out of all of them. Good little ship, can fight, haul, or explore at a whim. I did it all in mine. Or you can save your money and get a Clipper for 50 million nicely outfitted. A good all rounder that can also haul or fight well.