Yeah, I would rule that you get magic beast HD and +1 LA. Like 3-5 magic beast HD being feral creature. Also thayan gladiator is nasty as it improves his natural weapons and I myself would ban spellfire as it makes any char with high Con immune to magic (or at least spells which ask SR).
Anyways the formula for HD is max each pairless level and in paired levels it is 75% total result (so 1/4 less from max). With HD it go;
Now I know folks do not like count fractions but it's easy to count them. Just round it down and add +1 HP per 2 HD/class level. Meaning normal drows get +6 HP added after adding rounded down fractions.
Now some monk advice; I would recommend pick a monk supporting PrC as most things gained as monk in later levels are not worth it. Also by lore drow monks should focus to be assassin alike or special stealthy enforcers for matron mothers. Instead relying traditional rogue/assassin route monk is more straightforward killer. Some PrC's and reasons to pick them;
Dragon Descendant; Gives access to some nice constant buffs (and class levels stack for unarmored dmg, unarmored AC and unarmored speed). In 7th level gain kind of 'stances' which you can take as swift action. One gives immunity to fear, +2 attack and dmg and allows treat firsts as chaotic aligned and magical. Second one is suitable for assassinations (reduced penalty to stealthy checks while moving full speed and +2d6 sneak attack). Third gives ray attack. Fourth gives save bonus and DR/cold iron. In pinnacle could use combat reflex gain AoO vs attacks attempted vs you (and later on combine two stances). Plus some nice mental fortitude stuff.
Source; Dragon Magic. Asks to have dragon ancestor and blood in veins (dragongrace is cheap feat to get in).
Tattooed Monk (source; Complete Warrior): A lots good stuff in this PrC. You can get cherry pick from dozen nice abilities by your preferance. Need waste one feat slot for endurance but that is good for athletic sport stuff what monks are well known. You get 4 tattoos and each level counts stack for unarmed dmg, unarmored bonus and unarmored speed. Example tattoos;
Bamboo, Bat, Bellflower and Butterfly; Add +4 bonus either Dex, Con, Wis or Cha to last 1 round per level PrC 4/day.
Centipede; Shadow Walk 1/week, can be useful to travel fast in underdark and not harassed most creatures.
Chameleon; Alter self 4/day lasting 7 hours per transform. Do I need remind several nice shapeshifted forms you could take boost yourself in battle or answer situations (need flight? Become a winged elf. Need disguise? Change to person you want).
Crane; Immunity to normal diseases, poison and aging.
Dragon; Able breathe fire as if taken elixir of fire breath 4/day
Falcon; Immune to fear, allies 10'ft gain 4+cha mod bonus saves vs fear.
Lion; Able smite 4/day, +4 hit and +7 to dmg.
Monkey; Gain +4 bonus sneaky rogue skills (climb, escape artist, hide, move silently, tumble, disable device, open lock, sleight of hand).
Ocean; No need to eat, sleep nor drink.
Scorpion; 4/day forces foe use lowest stat instead highest stat to affect attack roll instead Dex or Str. So ogre uses Int instead Str to determine does he hit or not.
Spider (needs stunning fist); Makes a contact poison attack with save DC 10 + PrC level + Con mod. First and second dmg is 2pts con dmg. Uses stunning fist charge.
Tiger; 4/day allows for 7 rounds +1 bonus attack rolls and +1d6 extra dmg per hit.
Tortoise; 4/day can use skill as if monk had 7 ranks in it (even skills which cannot be used untrained).
Wasp; Can 4/day cast haste on self, lasts 7 rounds.
White Mask; Immune detect thoughts, detect lies and any method determine monk's alignment. Also +10 to all bluff checks.
Now some feats;
Defensive throw (dodge, improved trip, improved unarmed strike, combat reflex, complete warrior); Foe which you have dodge misses you, you trip him as response and it counts as AoO spend.
Paralyzing Fists (Wis 15, Stunning Fist, Drows of the Underdark); Hit twice in row with stunning fist and foe failing twice save vs it. Foe is then paralyzed for 1d2 rounds.
Note; There is Freezing Life-Blood feat from Complete Warrior which allows do paralyzing attack directly (spending stunning fist) and lasts 1d4+1 rounds, but has high +10 BAB requirement.
Pain Touch (complete warrior); Round after target recovers from stunning fist they are nauseated for 1 round (automatically).
Roundabout Kick (complete warrior); Crit with unarmed attack and you get to do additional unarmed attack with same attack bonus (very situational but if dice is lucky it hurts a lot).
Weakening Touch (complete warrior); Spend stunning fist, no dmg but foe suffers -6 to Str for 1 min (a automatic debuf?!).
Now a variant note; In PHB II there is variant for flurry of blows called Decisive Strike. It allows do single attack as full round action (two attacks in 10th or 12th monk level) which do double of normal unarmed strike dmg. Why care? It doubles any dmg adds to these attacks! Double str, double power attack bonus, everything.
Also tip; get martial gloves from magic of faerun. It allows use gloves as unarmed combat aids which you can enchant to give weapon enchant bonuses to your fists (including +1, +2, +3, flaming, unholy etc). Costs a base +1 pair a 4,000gp. Advance working same as adding enchants to two weapons.