Once I can get Firestorms, interceptors go bye bye. Skyrangers are keepers though, albeit they cost half a mil a month they do have an incredibly long range, they are the best way of finding alien bases, they can transport HWP's/More soldiers then the Lightning.
Engineers are an easy source of income, motion scanners are good first, then laser cannons, then fusion ball launchers, although laser cannons will be the majority as they are relativly low level research. Its good just to have a base consisting of a few workshops (and labs for research) just producing laser cannons to make cash, however nearing the end of the month dont forget to transfer the engineers and scientists to another base to save money by not paying thier salary
However selling Alien Artifacts will get you a lot of initial income, just be sure to keep one of every item until you have researched it, the only alien artifacts you should not sell should be Alien Alloys, Elerium-115, Alien Grenades, Heavy Plasma Clips, Heavy Plasma Rifles, Blaster Bomb's and Launchers, Stun Launchers and Bombs.
You might want to keep a Mind Probe or two but they sell for an incredibly amount of cash each (around 300k if I remember rightly). You can also sell excess Stun Launchers, mabye even Blaster Launchers, but you should not sell the ammo as that is expensive and can be hard to come by.