Hi
Ok, first, automatic importing of products from Magento is generally not a problem, but I need vendor list to tell you for sure, and make a right price and timeframe for you.
Out of stock is possible in almost the same way as the scraping the products, running scraping script as a cron job every hour or so, and updating in stock or out of stock information
Third part also eed a vendor list. As I got, you don't need automatic buying, you need automatic login and shopping cart filled with all purchased products from this vendor, when admin will click a link on backend? Again, need a vendor list there.
Best Regards, Yura