Sound like you've got yourself a huge bargain there, well done.

If you don't need to sell the Rega straight away for the cash then I'd make a new arm board for the Systemdek/Rega combo  ( Use MDF, or Conti-Board from B&Q) , then fit your RB250 and see what it sounds like.

The Arm geometry for the Linn arm and the Rega arm are  different BTW, which is why i suggest fabricating a new Rega arm board. All you need to do is get the corrrect thickness ( or very similar) of material and then draw around the old one, then cut it out. Obviously you will then need to mount the blank armboard and accurately mark out where to cut the new arm pillar hole , I think this needs to be something like 223mm from the centre spindle, however do some research and look into this as i'm not 100% about this measurement

Systemdeks have been sold with both Linn/Systemdek arms and Rega arms so both will work with the correct armboard.

I used to own a iiX a few years ago , however mine was fitted with a Systemdek 'Profile'  Tonearm... I think they are great decks and look to be very 'tweakable' if you like fiddling and meddling...