The Essex motors are basically Ford copying GM's idea of a large displacement V6 in place of V8s. It's not a direct copy of the Buick 3.8 but its obvious Ford saw the merit of the Buick motor, took the GM motor apart and took very good notes.
But if you're going to do an Essex, you might as well just do a 5.0 V8. While the low cost of the Windstar motor is appealing, you might find yourself having to rig a lot of stuff up for controls and accessory drives. If you're using a manual transmission, you have to figure out how to make that work since there are no manual gearbox Windstars. Can you even get clutch components? If it's an automatic, you avoid some of those issues, but you do need to mate everything up. Not impossible but if you're having to swap transmissions, control systems, cooling system changes, etc. you might as well just take advantage of the more powerful V8 motors that will fit in the same space and have most of the engineering done.