One hint (that worked for me): Get User Agent Switcher for firefox, put on a N900 user agent and you can browse the Ovi Store just like it was from the phone. Do the buying procedure from there, and just transfer the .deb you get to the phone manually later.

I'm saying that cause seems most of the issues are with actually getting the deb and the weird errors that come up. By just transferring the deb later the installation went flawlessly here.

(ps. if this some how facilitates piracy, please delete the post, mods!)