If you’re looking at a Macbook… see here:
https://help.ubuntu.com/community/MacBook
for info on how well Ubuntu works with different Macbook models hardware.
Any particular reason you want a Macbook ?
I’m no expert on Mac specs, but didn’t the earlier Macbooks have a limited amount of RAM ? … IMHO you want at least 1GB for a full GNOME or KDE desktop… though it WILL run in much less, it will be much slower as a result… or you could run a lighter desktop, but then may run into other issues such as ease of setup and/or lack of support.
I personally wouldn’t bother with overpriced Mac hardware unless there was an overriding reason to… you’ll get much better spec for your money if you go for a standard laptop… even if I really wanted OS X (which I don’t
), I’d still either get a laptop that is known to work well as a “hackintosh”, and/or get a laptop with a Core 2 Duo (or i3 etc.) and run OS X in a virtual machine (VM), from what I gather you need hardware VM acceleration for OS X to run in a VM which is why the Core 2 Duo (probably work with any CPU that supports Hardware accelerated virtualization, but I’m not 100% sure).
Ubuntu/Ubuntu Studio (full GNOME desktop) might not be as light on system resources as you think, and more specifically with your stated uses of Audio/Video editing/production, you’re going to want the highest spec you can get for your money.
‘Specific’ laptop hardware, particularly when second hand is a bit difficult to recommend in ‘general’ terms…
An example…
‘Normally’ the rule-of-thumb is to go for something with an NVIDIA graphics chip, but then when talking about second hand, you might end up with one of the NVIDIA chips that were known to fail which you don’t need if it’s “integrated” onto the motherboard.
http://www.theinquirer.net/inquirer/news/1028703/all-nvidia-g84-g86s-bad
and
http://news.cnet.com/8301-13554_3-10020782-33.html
and many more articles all over the web at the time.
(I personally have seen MANY dead laptops because of failed NVIDIA chips from this period, and am still pissed at NVIDIA for this debacle, and the subsequent cover up)
All I can suggest is do your homework, and do a search for known issues with any laptop model before buying, or ask in this forum when you can be more specific about the make/model/hardware.
That said, Linux can be made to run on just about anything… just a matter of how much work is involved in getting all the hardware to work (such as integrated wireless, webcams, daft Apple extra keys etc.), and how fast it will run (obviously resource dependant) 