I have two, I can't live life without an air conditioner! But yes, a proper air conditioner is large, heavy and noisy....and pricey.
The cheaper, smaller ones are not proper air conditioners, they usually blow air over cold water or ice in a compartment and then blow the cooled air out into the room. They are not effective when the heat is up to 30 degrees or whatever.
A full air conditioner has a refrigeration unit inside it which is what makes it heavy and noisy - but effective. I usually buy mine through Amazon or Appliances Direct (www.applicancesdirect.co.uk) - in fact many of the Amazon ones come through these people. They have a larger range, check them out online; you can get the odd air con via Argos or B&Q but they are usually lacking in choice.
The best time to buy is out of season. If you buy them in January, prices are often up to £200 lower in price as no-one is buying them then and they have stock to clear. Once the hot weather comes you can pay £300-£400 or even more per appliance.
You need to get up to speed on BTU - this is the capacity of the air conditioner. A cheaper one will only be something like 7000 BTU and this won't really chill very much once it gets hotter unless you have a room the size of a cot bed. 1000 or 12000 are more common but are more expensive, they will make an impact on a normal sized room. However, the best ones are 15000 or even 18000, not often seen for purchase online. They will also be very expensive, some can be £600 in peak season. But they are brilliant when we get those 36 degrees days. Air conditioners are good buys though, once you get a good one they run for hours and don't often break down.
It is easier if you have sash windows as there is always a pipe that takes the heat out of the window, but any window can be blocked off with a large piece of thick card over the open window space, and a hole cut in for the pipe, and sealed around. If you know a builder you can always drill through the outer wall and have a pipe to the outdoors permanently fixed, of course.
I always fix very, very fine mesh over the open end of my outer pipe before fitting it in place, this stops insects crawling through the pipe and through my air con into my house! (Spider fear!)
As for types, I currently have Electriq for my two but have used Amcor which are also very good.