When you start a new game you have a set amount of funds and it is up to you what to do. You can buy a pre-made house and decide whether you want it furnished or unfurnished. Obviously you are limited to the houses you can afford. Or, instead of buying a pre-made house you can buy a plot of land and build your own.
Not too sure what you mean by buying any other pre-made houses. I'll try as best to answer. So, you can move from house to house. If you sim starts making money you can decide to move to a bigger house. You choose the move option either through the sim's mobile or pc and the game totals up how much money you have to spend. It will add up your free cash plus how much selling the old house will bring in. You can also choose to take the furniture with you or sell it. This is through a tick box and again the game will calculate for you your total cash spend when you tick or untick the box. Then you can choose your new house and move in.
In your home town you are only allowed to ever own one house/lot. To be technically correct you can only own one lot that you live at, and most people tend to just have one house on it with associated garden, shed, etc. However if you have the World Adventures expansion pack you can also own other houses/lots in three different countries, one in each country. You can't live there permanently however you can take vacations and travel to your other houses.
