There is no CGT for a property during the period in which it is held as your principal private residence so all else being equal, in the scenario you set out there would be a gain for CGT purposes of 1/6 of the increase in value (given the property wasn't your PPR for the first year of 6). So this would be a gain for CGT purposes of €33.3k.
In terms of the market value of the property, surely the fact that the house needs a lot of work means that the market value is not 20% more than what is being paid?