Flasher here: http://skeiron.org/tablets-dev/maemo_dev_env_downloads/
Image here: http://skeiron.org/tablets-dev/nokia_N900/
Guide here: http://wiki.maemo.org/Updating_the_tablet_firmware

Regarding the repos, that is an issue with extras-testing and extras-devel repos (hashsum mismatch error).
Open the catalogue, go to extras-testing and extras-devel, and change "distribution" from "fremantle" to "fremantle-1.3". Wait for the refresh, and you should be able to install apps normally then.