Based on the responses, I'm not certain the OP would be restricted to automating AutoCAD from the outside via COM. This could be done within AutoCAD, consuming the Excel content as data. Thats generally easier and faster than the former. The "do this in front of the client" routine where everything updates as the data is changed is a bit much as described; it would be easier to do the work in Excel, switch to an active session of AutoCAD and execute an update or refresh command which would read the new values in Excel and change the drawing accordingly.
Since code is the end product and not the starting point, I think the best place to start with is detailed evaluation of the various processes you need e.g. read Excel file, convert data to something useable (validate for bad info!), create or link to AutoCAD objects, modify AutoCAD objects, and so on. At this point, you should have a rough idea of the time budget required for the work. Those procedural notes can be expanded into rough code, which can then be tested to evaluate desired work processes, help determine where user input is required and in what form. The rough code can then be detailed out to get the final product.
As different interactions with AutoCAD are needed (e.g. create a line, insert a block) they can be investigated using examples here, in the ObjectARX SDK, and elsewhere.
