I would recommend the Berzerker/Thief as the strongest all around Fighter/Thief dual class due to the amazing power of enrage. K/T is a lot of fun though. I've done 274 damage with a single backstab.
Kensai only get a 2 point AC bonus. With 18 dex they will start with AC 4.
The ultimate best equipment you can get with a Kensai is:
Helm: Rose? Ioun Stone (+1) Personally I like the Pale Green though.
Ring1: Ring of Gaxx (+2)
Ring2: Ring of Gaxx #2 or ROP +2 (+2)
Cloak: Cloak of Sewers or imported Cloak of Balduran (+1) Cloak of Mirroring doesn't give AC but is VERY nice.
Boots: Boots of Speed (+0 but you want them anyway)
Shield Amulet: Base AC to 5 (50 charges and can be recharged by selling and re-buying)
Potion of Defense: Base AC to 0 (limited supply)
For a K/T I recommend single-weapon skill and Katana. This will give you another +2 AC and if you use the Zerth blade you get another +1 (Celestial Fury is better though).
So, your AC can be -5 normally, -10 with the Amulet, and -15 with the potions. With what I consider to be the best equipment you'll be at -2/-7/-12.
If you're working in a party, don't dual your K/T until level 13. This gives you an extra +1/+1 hit/damage, another Kai, more hp, and most importantly +1/2 attack which becomes +1 attack with the Boots of Speed. This allows you to single wield for the AC bonus and still get 3 attacks/round.