Annie - Don't think you need to go that far with system surgery.
First find a system you like (features & price) and get it. Next see what size the system has. If the drive is larger then you can use Apple's Migration Assistant application to move your stuff over completely to the new systems HD.
OK, that sounds simple, But... Do you have a system which you can access your HD from {Mac} Then you can put the system into Disk Mode to act as an external HD and connect it to your new system via a firewire {best}, Ethernet, or USB cable between them.
What if I don't have a system to put it in. You could get an external HD case to house the HD (Firewire or USB) and use Migration Assistant that way. HEre you have a bonus as you could then use the HD as a boot disk to do house keeping tasks like checking your systems disk or defraging it. Also you can hold anything important you don't want to loose as a backup.
OK, what if my drive is larger and I want to put it in the system? Sorry to say it may not be the best idea. The newer systems have a faster I/O SATA II Vs SATA III so you will loose performance and the newest systems have a custom cable and HD setup which your HD wouldn't match up with correctly (fans go into high speed mode). Seagate has updated their firmware on the newest over the counter replacement HD's to now work correctly within the iMacs so you may find it better just to replace the current HD instead (Vs swapping it).
Lastly, you should review the iFixIt guides for the systems you are planning on opening up to get an idea on what it will take to do it.