Yes, you are starting the wrong way.
You can not resell or offer for free things like "Forest house sketchup scene is free scene that on internet" and it doesn't matter if it is modified or included in a scene. You can do that only if you have permission from the author.
You are allowed to sell or offer for free only models that are modeled by you or you have the rights for.
That is what you had agreed to when you accepted the site terms and conditions.
As for your question about is Sketchup and Lumion enough, it depends on what is your goal. Enough for what - a coffee per month?