On the roundnose (Mk1) Multipla, parking sensors were always an optional extra, available for both the SX and ELX. In the 2003 model year they cost £399.50, so not a cheap option. I think the same is true of the squarenose (Mk2) but I haven't got a brochure/price list to confirm it.
While I've got the 2003 price list out, the full list of options was:
Metallic paint (£290)
Parking sensors (£399.50)
Side airbags (£250; standard on the ELX)
Leather steering wheel and gearlever (£105; option on ELX only)
10 disc CD changer (£500)
Steering wheel controls on steering wheel (£65, but I think only available if you also spec'd the leather wheel; option on ELX only)
Fridge (£250 and you lose the middle front seat)
Radio with single slot CD (£120)
Window bag & side airbags (£425)
Window airbag (£175 - for ELX only)
Double sunrof (£600; standard on ELX)
Climate control (£702.65; standard on ELX)
Connect (£800. Ouch!)
Navigator (£850. I have only ever seen one of these. A real collector's piece)