Simone,
HDV is difficult enough as it is, but upresing SD and mixing with HDV is exponentially more difficult. Without a great deal of experience and expertise it will be very easy for you to make mistakes that will be compounded and multiplied as you get farther along.
I would advise that you carefully consider letting the market and the end product dictate your path in post-production. What type of project is this? What will be the finished length of the project? Is it for TV, DVD, theatrical, etc.? Let those factors determine how you will proceed. HDV may seem tempting, but you don’t have to choose the HDV path simply because some of your material was shot on HDV. Editing in 16×9 SD is suitable for very many projects.
DRW