You are going to need a laptop capable of running your DAW so having 8gb of RAM would be handy, also you will want a good few USB ports so you can handle your external audio interface, and external HD's for backups. With regards to plugging in synths, headphones, mics that will all go through the audio interface.
Another great place to start on this journey of research would be HomeRecording.com where you will be able to get lots of advice from amature's and pro's alike on everything you could possibly want or need.
Judging by your post, I could be wrong, you need to do lots of reading before you even break down you budget and start spending. You dont need the latest processor to run your DAW, having bucket loads of RAM will help far more, if you do that you are spending lots of money on something that wont affect your composition at all.
Even for basic listening I would still get an external audio interface, and on this one piece of kit alone you can spend from $150-$15000. Your computer is only one part of the chain, and when it comes to production its more like a web when you factor in headphones, speakers, mics, midi devices ect.
Read LOTS before you spend anything.. there are hundreds of great sites out there for you to look at regarding recording, production, home studio's, recording live. Beware of peoples bias in advising what to buy, find a local dealer and maybe pay a bit more for local service.