Hi! There's a book series called "How to cheat in Flash CSx" (x is your version) which will show you a detailed professional approach to character animation as seen on Cartoon Network etc., made with Flash only. The author has real world experience from this, which proves that it indeed can be done. What the book offers are many tips on how to work effeciently, with for instance the minimum number of mouse clicks when manipulating mouth shapes synched to audio, etc. And much more! Actually I recommend this book for everyone who works with Flash in a time constrained setting. A mouse click saved hare, a smart working method there... - it adds up, especailly in character animation I'd say. So: it will quickly save your day, your budget and even your health, if this is your work. The "cheat" in the title is not the dubious version, but more like 'smart little things'.
Then there's "Flash cinematic techniques" which focus much more on storytelling etc., but also on how to manage your project within Flash; i.e. create reusable elements.
Whether Flash, After Effects or any of the Toon Boom products are right for you depends. They are all used in this business. Flash has it's frame-by-frame methodology like a music score which some really prefer (although is has been freed from it reins to a degree in the last two version by borrowing from After Effects). Toon Boom are stronger on the (semi) hand drawn styles; in my experience drawing skills are required for it. After Effects offers much more in the video and graphics effects department, if a more 'organic' style is what you want.
Not everything that can be counted counts, and not everything that counts can be counted.