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.
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
2
crwdns2947414:01crwdne2947414:0
I've just swapped out my Hitachi 750GB for a Samsung 2TB SSD (AUS$850)
crwdns2934271:0crwdnd2934271:0 kermit crwdne2934271:0