All my demo code is developed in UltraEdit (text editor) on the PC and then I use Devpac v3.18 to assemble on the Amiga.
Use of Devpac seems to be a British thing (the makers, Hisoft, were a British company) as, in my experience, mostly British guys use it.
Almost all of the European demos coders I talk to use Asm-One.
At the end of the day I agree with Joloo - which assembler you use all comes down to personal taste. The code's the important thing - the assembler's just the tool to assemble it...
Get the Hardware Reference Manual for learning the Amiga internals. If you'd like to see some source codes that include the boring but necessary stuff like system takeover etc. then check the Coder's Heaven forums on EAB. You'll find a good few of my sources knocking around on there that you're welcome to. You can always PM me questions as well if you like - I'd be happy to help you out. :-)
Anyway, I wish you luck JJ - in my opinion, 68k coding is rewarding and fun. :-)