Okay, this grew way too long, but here it is:
Nephew of Mentor wrote:ThiefAttack Dice:1
Defend Dice:2
BP:6
MP:2
Equipment: Dagger, leather armor, lockpick set
to pick locks: Roll one red die. 4 or higher is a success.
to pick pockets: Roll one red die, success on 5 or 6. One Hp counterattack on failure.
For comparison, the Thief stats from Dragonstrike:
Hit Points: 6
Speed: 6
Atack: d8 (dagger)
Armor Class: 5 (leather armor)
Save vs. Magic: d8
Find/Disarm Traps: d12
Find Secret Passages: d10
Feat of Dexterity: d12
Feat of Strength: d8
Equipment: Lock-picking tools, rope, bullwhip, grappling hook, food, mirror
Hit Points and Speed convert with no problem, as you already noted.
Limiting the Thief to just a Dagger, Hand Axe, and Spear could work, but maybe you should make the Hand Axe and Spear available as equipment cards for North American players. Personally, I don't like the idea of a Thief swinging a Pole-arm (Halberd). The Spear is already large enough for the sneak. Can a Thief buy a Crossbow? I'd want one if I was playing this weak-combat Hero.
Hopefully, the Sneak Attack ability should cover the limitations of weaker weapons. Do you plan to use the (5) Sneak Attack cards? As you don't want a Thief to use larger weapons, I'd extend my idea about buying training with extra Sneak-attack combat dice to triple the damage potential of the original weapon. A Sneak Attack card could contain the following text:
Nobody is faster with the blade than you are. This card lets you roll an extra combat die when making an adjacent attack with your weapon. For every 150 gold coins you spend to increase your training with Sneak Attack, you may roll an extra Attack Die. You may roll no more than triple the normal Attack Dice of your weapon when using the Sneak Attack ability. Discard after one attack.
A Thief would start out with a 2-Attack Dice Sneak Attack using the Dagger. For 150 gold coins he could then either buy a Shortsword or an extra Attack Die for the Sneak Attack ability. Assume he buys a Shortsword. He then spends another 150 gc (300 gc total) for 4 Attack Dice using a Shortsword with Sneak Attack; next, a further 150 gc (450 gc total) for 5 Attack Dice using the Shortsword with Sneak Attack; and finally, another 150 gc (600 gc total) purchases training for 6 Attack Dice using a Shortsword with Sneak Attack.
Note that Sneak Attack can't be used with a ranged or diagonal attack, as the Thief needs to get up close and personal. A 6 Attack Dice attack can also be achieved by a Hero with a Battle Axe and either casting a Courage spell on the Hero or drinking a Strength potion. You could simply allow 2 bonus, static Attack Dice and leave the Thief's best offense equal to a Warrior's default offense with a Battle Axe--nice and simple. However, I'm left with the concern that most of the Theif's non-sneak attacks will prove ineffectual as monsters get tougher--that's why other combat Heroes can upgrade with better armor and weapons. (The Wizard also suffers from this problem.) My thinking is that the Thief should get a few superior, peak attacks to let him shine (like the Wizard), but average out with other Heroes throughout a Quest.
If you feel 6 Attack Dice is just too overpowered, you could amend the the pertaining text of the Sneak Attack card to "For an extra 150 gold coins you may increase your training with Sneak Attack and roll an additional two Attack Dice instead of one. For a futher 300 gold coins you may instead roll an additional three Attack Dice."
I could understand how paying for training upgrades could be unappealing. Another way to adjust Sneak Attack is stick with a basic two bonus Attack Dice, but increase the number of uses of the cards.
Nobody is faster with the blade than you are. This card lets you roll an extra two combat dice when making an adjacent attack with your weapon. If you roll at least 1 Black Shield, retain this card. Otherwise, discard it.
If my math is correct, a 3 Attack Dice Sneak Attack would see 5 cards extend to about 9 or 10. A 4 Attack Sneak Attack would increase to about 13 uses of the cards (Here my trials averaged only 10, so I have a math problem. My guess is 3 AD=8 or 9 uses, 4 AD=11 uses). This can actually swing quite a bit, as two of my 3-Attack Dice trials resulted in 14 and 15 uses of the cards, while 3 trials resulted in just 6 uses. I feel this luck factor can well enough model opportunities in combat while providing some fun.
Finally, you can just abandon the Sneak Attack cards and allow a Sneak attack bonus when an ally is opposite and adjacent to the monster the Thief is attacking. Or maybe just adjacent. You'd still need one card.
Nobody is faster with the blade than you are. This card lets you roll an extra two combat dice when making an adjacent attack with your weapon against a monster if an ally is also adjacent to and opposite of the same monster. Do not discard.
Just a thought off the top of my head, but maybe you could add a restriction for the Shield on the Sneak Attack cards stating
You may not be use a Shield on the same turn that you use this ability. I think a Thief is normally going to need the extra Defend Die, as he only has 6 Body Points. If you feel it is still too much, maybe the Thief is limited to a Buckler, usable just once a turn.
Another thing I think is missing from the Thief is the high feat of dexterity roll. You could put this into Hero Quest by allowing the Thief to be an expert treasure-hunter. He can discard a Hazard Treasure Card without effect if he rolls anything but a Black Shield on a combat die. This would mean printing this rule on another card, or adding it to the Hero Card. Maybe drop the pick pockets ability, I don't know how important it is in the adventures.
You're the Thief: nimble as a cat, as clever as a fox. Only a Thief can make Sneak Attacks, pick locks, and search an active monster for treasure. You also may avoid Hazards. To do so, you must avoid rolling a Black Shield on a combat die.
Of course, you don't need to put all this on the card if you plan to have the Thief rules for picking locks and pockets somewhere else. I like using the same dice as Disarming a Trap, the combat die: To Pick Locks, you must roll anything but a Skull on a combat die. You could also use it for Pick Pockets--you must aviod rolling anything but a Black Shield on 2 combat dice.
As for the Mind Points, I'd increase them to 3. That's even with the other expert Hero, the Dwarf. 6 Body/ 3 Mind isn't too bad if the Thief's abilities are enough of a perk. The BP/MP formula is really more what you'd call a guideline than an actual rule...
Keep in mind these are all just grist for the mill. Take what you like, reject what doesn't work for you.