Good plan...
This is a good idea (I don't know how I'd survive without at least one sever), but there are a few things to consider.
First off, you say you'd like to have your applications on the server. While this is possible in some cases, it isn't going to work out that well for a number of reasons. My advice is to just leave your installed applications on whatever machine you'll be using them on. You can of course keep the install packages or backups of your applications on your server though!
You'll also need some networking equipment - you have a MB, so I'll assume you've got WiFi of some nature going. If you just have an airport express or something with no physical ports, I'd recommend getting a switch or a router - just something that you can hook the server up to directly. You can absolutely have the server running on WiFi, so if that is how you have to do it - so be it. I just prefer having a server hardwired (faster, more reliable).
The next consideration is what kind of computer do you have available for the server? This is something serious to consider. The easiest thing is to just get another mac and use that - likely a powermac (or mac pro) of some nature because you can use internal harddrive bays (although, you can use external enclosures - consider getting a card that does eSATA for this if you have the cash). Networking mac to mac is also easier, although don't let that put you off doing linux/pc/mac networks - they really aren't that tough.
If those suggestions are just totally unreasonable because they are too expensive (which is likely given that you are just looking to serve some files) you can pick up a cheap linux box and use that. It will make a great server, but you'll have to figure out linux which can be a project in itself if you've never done it before. Check out Ubuntu for an easy distro to use, although everyone has their favorite one and some people feel very strongly about one over another...
Really at this point we need to know:
-What hardware do you have available to you (networking gear, computers, drives, etc)
-How much money are you prepared to spend ($300 could probably get you started with a cheap linux box and a router, but we can certainly help spend your money for you if you have more
)
-How much data do you have and what is taking up your space (ie. what kind of files do you have filling up your drive)
...Another simple solution is to just get some external enclosures with a power switch if your only concern is that they don't turn off. I use Vantec Nexstars and just put whatever drive I like in them. They work well with Macs and PCs, they have a power switch, and have both USB 2.0 and eSATA if you need the flexibility.
Good luck!