The stairs were changed for a reason - best to ask vendors why they put spiral staircase in because it's a strange thing to do if there is an alternative. If they say they just preferred spiral I'd be a bit but then it might be worth asking/paying someone who specialises in architectural joinery along to see if you could fit a staircase that would be acceptable to you and meet building regs.
Agree with lala that you need to get someone professional to take a look and see whether a conventional staircase could be fitted.
As the stairs could possibly be changed then I can't see it being a deal breaker. To me a deal breaker is something that you can't change like a garden being too small or no hall and no way to gain a hall.