Want to swap out HD for SSD's

Follow this IFIXIT guide: Mac Mini Late 2012 Hard Drive Replacement.

As to which SSD to put in: Either will work, you may want to look at going with a larger unit though as 256 GB is on the small side.

Are you sure you want to swap out the 1 TB HD here? Frankly, I would recommend you add in the SSD as a second drive. Then prep it up as your boot drive and leave the HD were it is as your data drive holding your work, music & videos. The SSD would hold your OS files & Applications.

Follow this IFIXIT guide: Installing Mac Mini Late 2012 Dual Hard Drive Kit. You'll. Need this kit for the needed parts: Mac Mini Dual Drive Kit. Even with the dual drive setup I would recommend going with a 512 GB SSD. Besides the OS & Apps, the boot drive is also used for virtual RAM and swap file space as such you want to leave no less than 1/4 of the drive free and if you only have 4 GB of RAM you should have more space available. You may want to add in additonal RAM after you have added the SSD. I would have at least 8 GB.