I have been to Lagafater several times. I recommend it thoroughly.
The first thing to appreciate is the remoteness of the house. The sense of isolation, the true darkness of the night sky and the complete absence of traffic noise all contribute to the feeling of being far from the maddening crowd. Of course that also means that you can make as much noise as you like without disturbing anybody else.
The house is an Edwardian time-capsule that has been tastefully modernised, so you can have log fires for warmth and an Aga to cook on, but backed up with modern heating and kitchen fittings, gas rings etc. There is even a dedicated drying room for wet clothes etc. The house is large enough that even on a rainy day, everybody can be accommodated in the downstairs rooms with no feeling of being cramped together.
The estate is true wild moorland, so proper equipment is essential (walking boots, waterproofs, maps etc.) I also recommend a recreational/waterproof GPS that can survive being dropped in a bog, unlike an iPhone. Note that weather forecasts will be for lower levels, e.g. Stranraer. The house is considerably higher than that, and forecasts can be slightly askew.
Other things to do:
Find the last remains of the aircraft that crashed in 1945 at Grid ref NX 13265, 77901.
Retrace the old road from Shennas to Glen App.
Short walks: upstream beside the Laggy burn, or down the track to the ruined Strabracken cottage.
Other local attractions: Luce Abbey, Castle Kennedy Gardens.
Further afield: Luce Bay beach, Culzean Castle (National Trust for Scotland), Portpatrick town.
It will pay to be organised: Decide the meals in advance, and have someone do as much of the shopping as possible on the first day.
The nearest major supermarket is in Stranraer, 40 minutes away by car. Of that journey time, 30 minutes is on single track roads with passing places. Sheep roam freely over the road in many places, and as you get closer to the house they get less familiar with traffic. This can get ridiculous, with mother & lamb panicking and bolting away at the last minute, down the easiest route which will be the road you are driving along!