Hydraulic pumps

[mspivak@hyperdesk.com (Spivak, Marat)]

     I am faced with a replacement of the hydraulic pump in my
     '90 100.  I heard somewhere that most recently rebuilt ZF pumps 
     have major design changes improving longetivity.  Is this
     true?
     
     The lowest price I was able to find was $ 225 from MAAS Auto
     Parts (I will have to return the old core).  The part number
     they gave me is 034 145 155 DX, and they claim theirs is the
     latest style pump.  When I called Shokan, they gave a different
     part number (8601 955 114W), which they claim is the actual ZF
     part number.  Their pump is $240.
     
     Could someone please enlighten me as what I should look for in a 
     replacement pump, or which vendor is best to deal with for this?
     
     Thanks in advance,
     Marat
     
     '90 Audi 100
     '90 VW Corrado G60