My recommendation is this 840 Evo samsung SSD. It is the best SSD in the market right now, and it will work with an Optiplex series. As a proud Optiplex 755 owner, I have looked into this. I think though, all you need to do is just install the OS on it, don't go for it as a pure storage device. If you only need it to install an OS (operating system) then a 120 gig should be enough. If you want the OS and a few programs as well, then 250 will be good. But keep the mechanical drive in it, as a storage drive.
My recommendation is this 840 Evo samsung SSD. It is the best SSD in the market right now, and it will work with an Optiplex series. As a proud Optiplex 755 owner, I have looked into this. I think though, all you need to do is just install the OS on it, don't go for it as a pure storage device. If you only need it to install an OS (operating system) then a 120 gig should be enough. If you want the OS and a few programs as well, then 250 will be good. But keep the mechanical drive in it, as a storage drive.