1) There are no drivers for the 5155 at http://zone.ni.com/idnet97.nsf/browse/
(but perhaps someone beside NI has programmed a driver)
2) The same type of instrument from the same company will likely have a similar command set. The drivers for the 4000 and 6000 are Plug and play driver and not IVI drivers, so modifying them would be easy.
3) I would first call LEM tech support and ask what the difference between the commands sets of the the 5155 and 4000/6000 are. If LEM tech support is unable to give a good answer, then I'd try to get the manuals for the 5155, 4000, and 6000 to just compare the command sets myself. If they are very similar I'd just modify the old driver. If they are very different, I'd just make I/O assistant VI for each function in th
e manual, and I would have a low-level driver. Once you have this low level driver you can just combine functions to make higher level VIs.
Hope this helps!