It's an option when Windows first starts. Press the F8 key the second windows starts to load and you should get a menu with Safe Mode as an option on it.
I wouldn't buy a prebuilt external drive, just a generic 5.25" usb/esata enclosure and then a decent normal sized drive.
I'm a more of a desktop (well, full tower!) pc guy myself (although laptops have their place), but it depends on what you want to spend your money on!