How to prepare computers for donation

Standard practice depends on how good a wipe you need.

Fast wipe: Write one pass of zeros across the whole drive.
Thorough wipe: Write alternating passes of zeros and ones across the whole drive at least twice.
DoD Wipe: Write multiple (I believe the standard is 7?) passes of alternating ones and zeros across the whole drive.

A tool like dban is probably the best way to accomplish this on a large number of systems.

(Note that this assumes traditional (spinning magnetic) hard drives. SSDs are Different and Special)

Re: the OS, Typically I turn over the media and license keys to the organization I'm donating to, but leave the machine blank in the state it was after the wipe was completed. This lets the recipient decide what OS they want to install and go about it however they wish (and if the machines wind up in a technical-training setting they may want their students to install the OS themselves).

If you install an operating system that requires a license (Windows) ensure that the license is with the machine (this covers your legal posterior).

For wiping data I would use DBAN http://www.dban.org . Making an image of three diferent models its tricky. I know that Acronis had a tool for doing image of diferent configuration (image without drivers). I would simply do images of three models with clonezilla.

Refer to my question here on security.SE. To reimage them I suggest using a solution where you use TFTPboot for windows. Your computers will then just boot from a prepared image over the network (so you don't need to put disks in every single one of them).