As a thief who utilizes many pieces for a variety of kills based on content difficulty I suggest you go /DNC sub (ease of curewaltz / haste samba / and an Erase like WALTZ are nice (no mp required)
Weapon suggestion for offhand is the
Jugo Kukri +1. This will give you lowest delay on content when your just running through easy mobs as quickly as possible and don't need a high dmg offhand dagger. This dagger allows for greater TP gains when you don't have trusts to cap out haste period.
Also if you don't have the
Jute Boots +1 (18% movement Speed) get them , as it will help out on the runs switching between kill areas.
Rest of gear mentioned above are great recommendations.