We built an extension and I fitted a heat pump. Most units now are heat pumps than cooling only. Dont buy cheap stuff like LG, its a false economy. All of my colleagues have Toshiba , just like I have. Oh and I work in air con.
Reason why I fitted it rather than run a radiator in there was because the boiler would struggle, pipework would be hard work , I got it cheaper than doing the rad install and its a room we double celetex insulated , much to the inspectors surprise , so it isnt used that much. It is the room the dogs sleep in , so when it does get hot, we can stick the air con on, when we are at work, which they love . Dont ask the size of the room, as its triangular ! but I fitted a 3.5kw unit which does work well and can actually cool the lounge next door easily. It runs on a 5 amp fuse and get an inverter compressor unit , as it will save money with power useage. Units from diy chains are usually chinese cheapies, which have no spares or technical support , so potentially cost more with replacing more frequently.
Its a great bit of kit, so make sure you look at Toshiba, Daikin or Mitsubishi Electric . Mitsi Heavy Ind is usually cheaper and a different company , but spares are awkward. Samsung is new to the market, but finish and quality isnt as good as the ones above. LG are cheap , will work, but not as long as better ones.