I don't think there is any risk to adding the extra HDD to your laptop, however you may have trouble finding room for it to fit. I know gaming laptops often times leave an empty drive bay in case you need to add some extra storage. as far as wiping your HDD, i would just go through and find all the important files that you need, and delete the rest. What type of laptop is it?
No worries. You don't even need to "wipe" it. It just needs re-formatting. But since it probably has hidden partitions on it from being a boot drive, chances are some data will remain with a simple format.
I would open a Command prompt in admin mode and type "diskpart". Once the cursor reappears, type "list disk". Then see which is the disk you want to wipe/format and type "select disk x" (where x is the number of that disk). Next type "clean". Once done, type "exit" a couple times to back out of the command prompt.
Go to Disk management in Windows and format that disk for use.