This game seems to be a marathon, not a race. It seems you level up quickly at first, but as the game goes on, with only one hardward store, etc. , it takes a while to get the parts you need. I've stopped adding houses and try to just level up what I have for now and keep the Sims happy to earn extra taxes.
Someone posted in another thread about bulldozing housing and building new houses to make money quicker. You might look at that thread and try it out if you think it might work. I am debating doing that. If I find the thread later, I'll edit this post and link it.