I've bought a few properties over years but I've never sold any of them so I don't actually have any property sales experience but here's my opinion for what it worth!
I would say that a furnished property should definitely help to sell the house quicker but it's unlikely that you would recover all the cost of the furniture. You could be putting some buyers off if they don't like the furniture, so I would suggest that you present the furniture as optional.
On the other hand an investor may be happy that the property is ready for renting straight away.
If I was buying a house I would be more interested in the quality of the fixtures and fittings and tiling/flooring if any.
I'm not sure about selling off the plans or waiting until it's built. Why not try sell it while it is being built and see what interest/offers you get?
I would suggest that you contact a number of local estate agents and see what they advise.
im buying a "builders finish" house at the moment. that means no paint, sanitary ware, kitchen, fireplaces etc. to us this level of finish was second best thing to building from scratch ourselves (we gave up looking for a good site) and gives us a lot of scope to use our own ideas and make modifications . if it had been completed we would not have been remotely interested.
With regard to furnishing, ask the question "would I pay for this?" e.g. for the make of kitchen appliances, type of floors etc. Buyers may not appreciate the value of any add-ons you include with the house.