OK, I assume you have looked at your stepper. On some S60 & S80 you are able to use the £48.00 Stepper. Some early and some 2003 cars have this. The first 850 & V70 Bi-Fuels also ahd the cheap stepper. Have you checked? If so, you are unlucky.
You are left with the 'Volvo only' version. Identified by the flat top. It sounds like you know what they cost (usually 5-6 times the price).
I know these systems well. If you buy an unmodded (used or new) stepper from Koltec/Necam, you will see the same issues in repeat maybe 25-50,000 miles later. If not kept clean or left unfiltered, much sooner. Do NOT buy the complete dizzy, unless you are lazy and don't want to clean it, or rich. Yours will only need cleaning.
My steppers require you to pay a 'core' charge set deliberatly high to be sure I get your old stepepr back to allow me to recondition and fix the Triac that ails these. You also get my tech support (very useful when it all goes awry) and a guarantee that should my diagnosis and info be wrong, your money less couriers, is returned. You will not get this last point with other suppliers because they are 'box-shifters' only. My background is 35 years in electronics. If you blow a stepper I can usually repair quickly and easily. I am sure I knwo more about the Necam system that anyone in the world outside of Necam. LPG specialists just mindlessly swap modules and components until something works. [Where might you think I get my supply of so called 'no good' parts from?
]
For added compliation, (read expense) the £48 stepper is not matched with the ECM in your car. The stepper motor is at the route of most of the bad-mouthing of what is otherwise a very simple system that works well, gaining a poor reputation undeserved. Clean it, correct the stepper and all works.