I don't have the money to make it .com right now, so I was wondering if I can get the free one, write a whole bunch of content on it, then later upgrade to just .com without losing my content.

Keep in mind even when I switch the domain name to .com I still want to be using wordpress, just that I don't want it to show in the name...

Yes you can,but you will be charge if you want to buy dot com domain to your wordpress.com site,you can also check through the wordpress.com dashboard,I think it will cost you $18.