Would it be wise to buy extra PATA drives as spares for notebooks while they are still available, or can such interfaces be changed to SATA with ease and comparable cost?
Are you going to use the hard drive for desktop or notebook computer? The PATA to SATA adapter is available for under 20 bucks (just do a little search), but it requires a little more installation space, which I'm not sure available if you are using it for notebook.
I was specifically thinking about notebook type computers. In our family, we have an IBM Thinkpad, a Dell Inspiron, and a UMPC sold by Vye Computers. I can't even get into the UMPC to see the hard drive, and their tech support informed that if the drive ever failed I would need to return the computer to them for service. This computer was quite expensive when I bought it a few years ago, probably cutting edge at the time.
Since SATA seems to be the universal standard now, it would probably be wise to invest in those connections. I'm not sure how the cost compares to PATA options, but I'm sure SATA will be fairly cheap in the future, if it isn't already. SATA drives are definitely faster, too.