Workhorse P chassis are not the same as the W chassis. Allison transmissions were only used in the W series. Ford chassis will all have the V10 engine of various designs. The early ones had some problems and the later ones are fine. Since you are looking at used, all of them are out of warranty. Both Ford and GM/Allison parts are available everywhere.
Go for the first one that has the floor plan that you can live with and just know that the engine will still be running long after the house becomes obsolete.