Provided the motherboard supports it (which any board made in the last 5-6 years should) booting Windows 7 from a USB connected drive will work, it's just not practical as an everyday solution for running Windows, given that it's bandwidth capped at 60 MBps.
Any eSATA adapter should work, as Jonmor said above, just make sure it is connected via the ExpressCard slot instead of the older CardBus slot.