My advice would be, dont sell the Cobra Mk3, its the ship that will give you at an early stage of the game, the best all round ship, to do everything.
Keep doing what your doing, and save up the funds to get a Viper Mk3, kitted out ideally to maximum, and go hunting. Some modules (although I admit not many just sensors and power dist unless youve downsized) and your guns can be transfered back and forth between the two ships freely, as required.
Alternatively, go with the Viper Mk4, most of your Cobra Mk3 modules will just transfer over to it, its a popular little ship for combat, but costs a little bit more, at around 300k. But as you can probably use all the modules off the Cobra (And just transfer them back and forth), it maybe the cheaper route. Its a little bit "heavier" to fly than the Viper Mk3, but has scope for being way more tanky, and can be engineered to be pretty lethal. It wont be as fast as the Cobra Mk3 though but see my PS below.
PS : You say faster..... the cobra Mk3 is one of the fastest ships in the game.... so "faster" may not be achievable. But the Cobra Mk3 has some pretty wacky hardpoint placement (in my opinion), so it can make for a poor choice in dog fights. A Viper (Either) will have better placed, otherwise identical hardpoints, 2 small, 2 medium.
PPS : Dont know if you have engineer access, but all the ships in the game will benefit from engineered equipment, and any ship can be made, faster, with Dirty Drives, but thats another post reply altogether.