What's your chosen investment strategy? Are you investing for - CF or CG in the short to mid term?
If its CG I would suggest not purchasing a display home because most are built in outlaying areas where the supply of new housing/vacant land far outstrips demand. There is no consumer competition to increase purchase prices.
Having worked on display homes in a trademan capacity, I would not purchase one from a quality perspective either. They are bashed up so quickly and there are a lot of short cuts taken that are band-aided & hidden out of sight the consumers can not see. ie roof cavity, service ducts, cupboards etc.
Sure, you can get a guaranteed rental for 2 years etc but you also have to make sure of the current weekly rents plus you are not just getting your own money back in rental income from the builder from paying an over inflated price in the first instance.
I hope this provides some food for thought.