I have not opened up a mini (I have opened 4th gen and older iPods), but I have seen some of the online guides, like this one
http://www.ifixit.com/Guide/Device/iPod_Mini
It does not seem too difficult for someone who is careful and good with small objects.
You can get such replacement batteries on eBay for less than $10 including shipping and the tools.
[For example|http://cgi.ebay.com/Battery-for-IPOD-MINI-4G-6B-1st-2nd-Gen-EC0003-EC00 08
W0QQitemZ190277166031QQcmdZViewItemQQptZOther_MP3_Player_Accessories?hash=item19 0277166031&_trksid=p3286.c0.m14&trkparms=72%3A1234%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1308%7C301%3A1%7C293%3A1% 7C294%3A50]
(I don't know anything about this particular seller)
Another interesting fact about the iPod mini is that you can supposedly replace the micro hard drive in the unit with a standard compact flash card. The shape and connector are the same.
http://geektechnique.org/projectlab/753/how-to-turn-your-ipod-mini-into-a-flash- based-ipod
I have messed with putting a 4GB CF card into a 4th gen iPod. You need to buy an adapter for that connection, but it does work.