My wife and I put a summer house at the end of our long garden a couple of years ago, facing the house.
My wife mainly uses it and it has a small sofa in it, a small coffee table, plus its is used as a "memory" store for all sorts of family memorabilia (family photos, holiday momentos etc. ). No electricity yet, though it is in our plans.
We spent quite a while looking and in the end got it made by a local garden centre who sell sheds, summer houses etc. We had a small "veranda" built as part of the summer house which has some plant pots on it which looks great.
We also put a patio in front of it, and a swing bench, and now we sit there far more than we do on the patio near the house.
Think about what you are going to use it for, is it going to become an outside office for example?
If so you need to spend more on it as it needs to be able to withstand the cold in winter. That of course pushes the price up. You can buy "outdoor offices" at garden centres etc so do go and look round.
Make sure you get one of a decent size, it is amazing how it soon fills up with seating, tables, shelves etc. We also store things like deck chairs and so on in it in the winter. I think ours is 10x8.
Often the ends of gardens are horrible with sheds, compost bins, tools etc so a summer house at the end (with space behind for all the horrible stuff) is a great way to brighten up the end of the garden.
Do make sure you leave space behind the summer house (6 or 8 feet), don't push it right to the end of the garden as it is amazing what you can hide behind there.