One point of clarification, BIM refers to building information modeling, a new method of tying all sorts of information to various elements in a model and is featured in programs like Revit and other 3D modeling software. What you are needing to get started with are blocks which are simply drawings of something like a door, window, a symbol, a titleblock or something more complex like a room layout or a standard detail. These are inserted into your drawing and can be moved, rotated, scaled or even mirrored as one entity. They can be copied multiple times in one drawing and if it needs to be revised, it can be edited and redefined with the changes being instantly applied to every instance of the block. There is or was a block repository here in TheSwamp. I can not find a link to it just now but will try again later. If I recall correctly, Intellicad will also come with a selection of blocks to get started with. There are also some collections assembled by other members that have generously been made available to all.
The rest of your interpretation of all of the input you have received seems to be spot on. If you decide to upgrade to a new release, you will be nailed for an upgrade fee, but there is generally no need to upgrade unless it solves some problem you have with the current program or promises a new feature you really want to have. Once purchased, it is yours to use as long as you wish.
There is one thing to think about that has not been touched upon yet - how to get hard copies generated. Sooner or later, someone is going to need your brilliant new plans on real physical paper. A little printer just won't do if you have plans on C or D size sheets. A plotter would do the trick, but they are expensive to buy and worse to maintain. Supplies can really eat up your profits fast, especially when you are trying to figure out how to use the thing. Unless you can find a great deal on a used one, it may make more sense to work with a local vendor to create paper copies of your files. A friendly engineering shop or reprographics store would be good candidates, even a Kinkos store may have what you need.
Good luck, it looks like you have a good start at a grand adventure!
Well . . . some others have made some of this redundant . . . so be it, I will let mine stand as written