The 45 is the guide number. They're all hamerhead, and basically the same size and shape. They all use the same battery packs which slot into the base of the handle - we've got a new 45CL4 and then picked up an old 45CT1 with loads of packs, and a charger. There are ni-cad packs or there are packs which take regular batteries - watch out for older ni-cad packs which die, and need to be re-celled.
The CL vs the CT model also affects (apart from being newer) the ability to work with the various 300 and 3000 series camera adapters, so they can act as dedicated flashes [it's pretty complex, so I'm not sure of allt he details]. If you're happy setting the aperture manually then all will be fine with either. However watch out for very old CT's they have high voltage sync which is not good for your camera (check Metz website for serial numbers). However my CT puts about about 15V and is fine.
The head can swivel on all models (I think), the 45CL/T4 have a secondary flash so when you point the main flash at the ceiling, there's a small fill in flash still pointing directly at them.
There are other more minor differences in exactly what setting you get - the 45CL4 has a low power "Winder" mode for use with motor-drives, and uses an led rather than neon power indicator.
Provided if you don't need the automation, and you check for safe-syncing then the older ones basically do the same thing. However the batteries and capacitors can die, so check that its all working properly before parting with cash.
Ian