welcome to amiga :-)
no you don't need a power cable, the 2.5 inch disk take power from the ide cable.
here you will find some info about amiga 1200 hardware and hard disks too.
-edit-
here is a 2.5 to 3.5 adaptor if you want in the future to try a 3.5 inch hard disk
amigakit and
here is a 2.5 inch ide cable.
-edit2-
read
this too.
