I don't know if you are going to be putting a shower or bath in your conversion, but if you are then make sure the flow rate of the combi is sufficient for all your hot water demands - they're not usually considered suitable if you have more than one bathroom/en-suite. If you do go for a combi then consider getting a Zenex GasSaver or Superflow (
www.zenexenergy.co.uk) as they make condensing combi boilers even more efficient.
As an alternative you could try one of these;
http://www.baxi.co.uk/ecogen. Micro-CHP is starting to take off, and still qualifies for the Feed In Tariff. If you've got a fairly large and relatively old house then it might save you a fair bit and even give you a bit of extra income (9p/kWh for each kWh it generates from the FIT and another 3p/kWh for 50% of the total kWh it generates from the export tariff).