To answer your question, if you purchase the software from someone who has upgraded, they would be SOL if they ever had a need to reinstall. Also, depending on their original license (if it was OEM, academic, etc.) it may not be possible to "legally" transfer that license to you.
That said, I would recommend Office 2007 as the product of choice. The Office Home and Student version is almost a steal at $149 list price (you can usually get it between $130-$149). Not only is it a full version, and cheap, it can also be installed on up to 3 PC's, instead of just one. The drawback is that it only has Excel, Word, Power Point, and OneNote. No Access, and most importantly no Outlook.
If you really want 2003, I would suggest giving OpenOffice a try. It's free, and 99.9% compatible with the traditional Office formats.