The Macs "just work". They are designed to be easy to work and reliable. The working phrase is "you get what you pay for", but Macs are not that more expensive, once you do a true comparison and factor in the software. My time is valuable, and I don't want to have to worry about a tool (the computer) that may fail for one reason or another. I have been using Macs for music since the beginning days of midi, so I think I can speak from experience. My main computer is an old dual G5 Mac tower, with another 7500 RPM int drive for audio recording and a Phonic 18 mixer / Firewire interface, with a FW 800 back up drive and a secondary FW 400 ext. drive that I use for archive and moving big files to other computers. And a USB flash drive, too-, and it does everything I need, including running vids for soundtrack production and music creation. All I do for maintenance is to run disk permissions and a free program called Onyx, every month to keep it cleaned up. I also have a PC laptop that I use, but I have it locked down tight and never go on the internet and browse, or download anything but updates for programs I already have. It works great, and is reliable, but I am very careful with it, to make sure it won't get infected. YMMV.
Almost all firewire interfaces do not need a driver, and the Macs "see" them when you plug them in. Important: Macs don't get viruses (they are written to infect PCs), so there is no need to slow down your computer with an anti virus program. I have been using Macs since the 80's and have never gotten an infection. And Garage Band is plenty enough to get started recording, and it's free and included.
I like the 13", but you will find quick that no matter if it's 13 or 15, you won't have enough screen space to work. I have 2 19" and that is not enough sometimes. You'll need an ext. monitor, but the Mac will, once again, see it when you plug it in. When I get Cubase going, I have a main window, a mixer, a score edit screen and a plug ins screen all fighting for screen space. I could use a pair of 24's easily. But get a 24 at least.
You'll need an ext. hard drive for audio recording, once you get into tracking more than 12 tracks at a time. The int. HD just can't do that and run the OS and the software at the same time, and keep up.
The USB audio interfaces are good for doing a couple of tracks at a time but that's about all they will do. Get a Firewire interface if you're serious.
The best thing to get is a Mac with a Firewire 800 AND a 400 port. One (800) for the audio interface and one (400) for the ext. Hard Drive. If you put a FW 400 and a FW 800 on a single port, the FW port will run at the lower speed. BTW, there are FW hubs available. Frankly, you don't need the latest and fastest Mac available.
I use Cubase 4, because I also do a lot of midi and score / chart production. A lot of people use Pro Tools, but it is way over priced for what it does. IF you use PT, be careful about assuming what you can move to another computer- sometimes it won't do it, the way PT says it will. It's not impossible, just don't assume it will work - there are other ways, just like going to and from PT and Cubase. But I use Garage Band a lot, because it is simple and to the point, and I can always transfer to Cubase if I need to. One other thing to spend money on- a good transparent 32 band EQ and a good reverb / room emulator program. Maybe a good guitar amp emulator, but Garage Band and Cubase have them built in, and they are decent to start with.
ANDFORGAWD"SSAKE BACK UP ALL YOUR FILES AND YOUR INT HARD DRIVE!!!!! I strongly suggest getting another ext. Hard drive, one of those cheap USB drives will do, and run a back up every day. Trust me, from the voice of experience (I had and ext. drive fail on me) , and the crying shoulder from friends who failed to back up (especially a certain PC user who got infected and had to do a wipe and reinstall - idiot used his music computer to play internet games on, and D/L'd a virus), do not take the risk. All it takes is a dropped or stolen laptop, and you are scroooed.