If the only other development is charging £400k (and is sold), its obviously an area where peple are prepared to spend. In general (looking at places like Spain, Turkey) initially cheap places seem to have sold and rented, but when the recession bites, its these places that dont sell or rent whilst the rich are a consistent market always buying/renting.

I think if you're aiming for the £200k+ market then you need to make them as high spec as possible. But if you're renting them out this needs to be countered with durability.

How about identifying the ones you are going to sell and making them as high spec as possible and then lowering the spec on the rented ones. Then when you come to sell at a later date replace kitchens, bathrooms, flooring.

Going with the natural theme, perhaps more woods, stones, handcrafted shutters, doors, kitchens, would be cheaper than more expensive high fashion items? I think if it is 'natural' you can get a way with rustic, rather than shiney, sleek expensive. Solar power fits into this natural/eco type development and buyers may be prepared to pay a premium as its cheaper to run.

Anyway, just my opinion (as a buyer and renter). Good Luck!