Buying Building Land in Northern Ireland

I am currently considering buying a building site in Northern Ireland with approved outline planning permission already in place. The asking price is £170k and I have a deposit of £140-150k. I would appreciate some advice on the best way to proceed. Would I be best to look for a self build mortgage now incorporating the money I would need to buy the site and build ( £20-30k for site purchase and £150-160k to build house) or to look for some type of finance to cover the cost of buying the site and take out a self build mortgage before I start to build. As there is only outline planning approval it would be 12-18 months before I would be in a position to start to build. I have no mortgage etc to pay every month, I am currently earning £32k per year and my partner £15k per year so we could potentially pay off any short term loan quite quickly.