I think the new devices and new features of Live 5 make it easier than ever to use without the need for 3rd party plug-ins.
Especially if you have Operator as well.
Personally, i find myself being able to get work just fine only using Live 5 + Operator, Hydratone from
www.tritonedigital.com , FinalPlug from
www.wavearts.com , PSP MixPack from
www.pspaudioware.com , and Automat (FREE) from
www.alphakanal.de
All the plug-ins i mentioned are $149 plug-ins.
I own alot of other stuff from Native Instruments, EAS, and PSP but i hardly ever use it as regularly as i do the plugs i mentioned above.
The included effect and instrument devices in Live 5 are pretty capable, and you also have free plug-ins like the AppleAU's that are included in OSX, the Fish Fillets bundle, Supatrigga, LiveCut, Zoyd, and so on.
Hardware wise, i would go for a 15" Powerbook.
the 14" iBook still only has 1024x768 resolution, and for doing design stuff, you will want the higher resolution screen.