Get one that doesn't require and additional power card. The smaller models which use the laptop sized hard drives get the power from the USB cable of the computer when you connect it. It ends up making it much more convenient in the future and they're very portable.
There shouldn't be any problems with compatibility. In the worst case, you can format it as FAT32 which is what most USB thumb drives come formatted in.
I have the Toshiba below and haven't had any issues:
4 stars in over 500 reviews.
The WD passport is a more common option and also has mostly good reviews:
Here are all the Amazon reviews that have the word "Mac" in them. Looks like it's good to go: