Welcome to A.Org and Merry Christmas (nearly)!
A1200s are quite easy to expand. Inserting a hard drive just involves removing a few screws to get to the innards of the machine. Any old 2.5" laptop hard drive will do, though there are some issues (fixable) with drives larger than 4Gb.
Adding RAM is a case of plugging an expansion board into the "trapdoor" slot underneath the machine. You can get RAM-only boards, but your best bet for price vs performance is probably a second-hand 68030-based board, which will usually contain a standard SIMM slot and an appropriate SIMM.
If you really get the "Amiga bug" and decide to go further, the sky's the limit (if you have a big enough bank balance!) and you can add a 68060 CPU and/or PowerPC, SCSI-2, graphics cards, interfaces to use PCI cards, etc, etc...
Have fun!
- Ali
