Advice on "Custom Building" an DELL OptiPlex

I'm about to get about 3 or 4 DELL OptiPlex (I don't know the exact model) or some sort of Dell Desktop Computers. Would I be able to put either Windows 7, 8, or 10 on any of the Dell Optiplex computers?

What would make a custom build with this computer better, and what should I buy (also, how much will it cost.)
These are coming from a goverment office so, the hard drives will be pulled (no biggie, i have about four )


Thanks in advance,
Cross

The most common upgrade are > CPU - Memory and HDD/SSD
Depending on how Old the PC's are and what comes inside them - will make your choices better understood..
I would think being from the government - they are very basic, baseline PC's as they most likely where connected to a server and do not have much thrills about them.. Would even guess they where XP machines, with older hardware so even your choice of a windows upgrade may be limited

I forgot to mention.... the guy I'm getting them from usually maxes out his computers.

I have to agree with KYHI. I used to be a Network Manager for a government agency. The PC's may be in good condition or they may be "clunkers". Workstations generally tend to be, as noted, basic machines. However just depends on the application they were used for.

Again depending on the specific models, many older Dell's that were designed for XP have hardware compatibility issues with newer OS'.
I used to do support on the Dell forums and this was a big issue when Vista came out and many could not upgrade because of the hardware (there were those that did anyway but the systems basically "limped" along). Audio was one big issue with some of the old XP machines as Dell used proprietary hardware and the lucky ones that could upgrade still had to buy separate sound cards to be compatible with Vista (and beyond).

OK then. Thanks for all help