You need to workout if you are going to sell them and if so when. This is the method I use in the Glen Iris area when building a build of units/houses.
If you sell once completed:
Built them as cheap as you can... just put in a few extra's that people will see such as the following...
i would say $100 a week for the smaller room without a carpark and $110 for the bigger room.
$210 a week plus going thirds sounds like a win-win-win
Well done mate... also I would suggest that you have something in writting where they can only have a partner stay over 2 or 3 nights a week...
I would use artificial grass as you never ever need to worry about it. I have in a few rental properties and the tenants love it because it looks good all year round