M.K.
How exciting! I believe that car seats have a 5-year "expiration" on them, due to the changing regulations and product development advances.
Personally, I would consider changing out the bottles and nipples (due to the BPA issue). I think it's a matter of your comfort level of whether you buy used or new -- I'm sure either is fine! We should probably note that your first children are perfectly healthy using the original bottles, cups, etc., so don't feel peer pressure into buying the latest and greatest.
We use so much gear when it comes to our baby and I'll sure the items you have will be perfectly fine. The only item I can think that might have changed in 6 years are baby carriers (Baby Bjorn, etc.) that are designed to be better for your back. If you have back problems, this is something you might want to look into. Otherwise, I'll bet you're good to go!
Best wishes!