1. Make sure the home is designed for a basement set, (Perimeter frame). Otherwise the home will be diffifcult to anchor to the foundation. This is critical!
2. The bottom board material exposed in the basement is very flamible, be cautious.
3. The addition you propose for the basement entry is difficult to do without attaching to the home, which will void your warrenty if you attach it.
4. You might need (depending on any local building codes) an interconnected smoke alarm in the basement, difficult to interconnect them after the fact.
You are much better off buying a home designed for a basement!