I like your plan but I don't think a pattern on the bed wall is a good idea. The carpet already has a lot of pattern so I'd keep that wall a solid, dark color, from the rug. Add some sparkle like mercury lamps for the bedside tables for interest.

Low ceilings = flush lights. Before you purchase carpet get 5 or 6 agents round to value. Ask each for their opinion on what they think will appeal best. Ask them if you could get away with just laying underlay. For the wow factor make sure your budget is spent on items that you can take with you - think a huge 6fx3ft mirror, a bright modern art print, a mirrored console table etc. No need to splash out much just one item in a room or two to indicate wow. A patio table and chairs in the garden are also needed for the selling photos as they will help the buyer to see scale and the garden will appear more homely. A bowl of fruit in a chrome wire bowl in the kitchen will also add wow - stage with green apples and yellow lemons for example. So, to sum up the spend on 'wow' should be limited and only to what you take with you. As for the estate agents some of them do try very hard to give good sales advice so when you book them to value explain that the property is being flipped and you would welcome any ideas on how to present it. You may be surprised at what they suggest.

Could have each corner as a different season and have the colours of the walls merge into each other - so, for example, with summer, it'll be nice and bright and yellow and then merge into orange for autumn and then the browns and oranges of autumn will merge into the white and greys of winter!

Depends on which room it is! If you're spending a lot of time in there, a lighter colour is best because with warmer colours, it tends to bring down the roof a bit if that makes sense - what did you end up doing?