The only advice I can think of is keep trolling these forums, there's a wealth of information on there!
Also make sure you create a REASONABLE budget, the majority that I have seen have cost in excess of £400!
As far as macs go they can more or less be understood in the same way as a PC with the exception that they are not as easy to upgrade parts. However, external peripherals can still be purchased for example a firewire/USB sound card.
At home I use an M-Audio Firewire Audiophile for linking my laptop to the hifi, the sound quality is great. It has 2 x gold plated RCA outputs for connecting two amps and also has Mac drivers, the sound quality on this card is great. I don't think they are in production any more but you might be able to find a bargain used one on ebay?!
I spent alot on my car audio and am currently using a stock head unit... A budget to me isnt as important as making it sound right! I acually spent $9000.00 on my current set up and I think any thing under $2000.00 for a car puter is great IF IT IS DONE RIGHT!!!!!! I would say if I set a Budget I wouldn't spend over $3000.00
Here are pics of my Mazda and current set up...