You can do that using common events. When the swich "X" is on the house will be useable. Now you can make 3 maps, one for a big house, one map for a medium house and a thrid map for a small house. The money you need will change with the house size. So imagine. If switch "can buy house" is on and switch "big house" is also on, you will need to pay the money for the big house.(you can also do this with the medium size and small size houses)
Now you should do the furniture. If you wanna buy a sofa. Make a switch called sofa appears and when you'll buy the sofa you must turn on have sofa, so the sofa will appear. Now you must do this with everything.
PD: you don't need to make the character pay money like I said
I hope I have helped you!