Human, drakeling, and ratling are all solid choices for assassin. Dark elf is not too great, as the only thing going for them is their starting armor, which doesn't fully compensate for their bad toughness and terrible shop prices.
Outside of a handful of weapons that are rare or difficult to get, dual wielding is garbage. You lose a massive amount of DV from not using a shield, attacking takes a lot more time, and weight penalties restrict the weapons that are practical to use. Assassin dagger throwing is nice at the start, but the lack of stacking ammunition makes it not such a great long-term strategy.
For talents, the safest bet is going for the PV line (Hardy -> Tough Skin -> Iron Skin -> Steel Skin) and the speed line (Quick -> Very Quick -> Greased Lightning). Get Healthy first unless it's a month of the Candle character. If you want to improve your dagger throwing you can get generic missile talents (Good Shot -> Keen Shot -> Eagle-Eyed and Quick Shot -> Lightning Shot), and then also make use of more plentiful types of missiles.
For weapon choice, it's generally best to just use whatever is your current most damaging weapon, instead of choosing a weapon group you want to use beforehand. The dagger backstabbing bonus is neat, but you really need a reliable way to get backstabs (darkness or blindness) to make the most of it.
Other miscellaneous assassin tips:
- Talk to the village elder before the druid. You want the starting quest that allows you to get Healing.
- Get crowned early. Assassin has an incredibly good set of crowning gifts.