SATA II drives...are backwards compatible with SATA I controllers.
I think you have some misconception about what an external drive is. It's nothing but either a SATA or PATA hard drive...in a box/enclosure with a a USB connection. If anything, it is less upgradeable in terms of the future...depending on the enclosure used.
You can take any hard drive, at any time...and make it an external by merely acquiring the appropriate enclosure and relocating the drive you intend to use.http://www.tigerdirect.com/applications/Ca...Drive-Enclosure
My suggestion would be to just get an internal hard drive, the fastest that your system supports. Honestly, I see no advantage to using an external hard drive as long as I have the capacity to attach internal drives to my system. I can see the advantages for users in the business world.
Since you seem to be concerned about how long drives last...it might interest you to check the warranties on the internal vs external drives. I'm pretty sure that you'll find that internal drive warranties are considerably longer.