Does Watchers Keep increase in difficulty as you become more experienced? Also what sort of a match is it for a Paldin, Ranger, Thief, Cleric, Mage, Sorceror party, just back from the underdark with loads of cool forged weaponary, at about 3 million EXP?

Most of the random mosters adjust to your skill. There are however fix placed (quest-related) monsters (e.g. Demons, Golems,...) who are he same every time.
For your party WK seems to be challenging but not too hard (depending on your difficulty set of course). There may be some hard fights but nothing to really worry about.

You are low on tanks. You are facing some hard battles. Just watch out and make use of summons.

The first time I completed the game, my party consisted of a sorcerer, 3 mages, one cleric and a thief, and I got no trouble. It
all depend on your playing style.

WOW!!! Well thats something I didnt know... do monsters adjust to your level??? I didnt know that.

i run a paladin (undead hunter kit)
with these items for his use:
red dragon scale armor from fikkraag
ammy of cheetah speed
pale green ioun stone (life porposes)
paldins bracers (life porposes)
casomyr +6
robe of mirrors
boots of speed
girdle of bluntness
ring of gaxx
ring of regeneration

i have beatin and imported my char 5 times now
he has a 105% mr
100% fire, ice, poison
200% lightning (robe of mirrors)
i got natural 25 strength 25 dexterity 13 intelligence 24 wisdom 25 charisma
my character alone has no problems beating anything in the game (cept for demigorgon, melissan,chromatic demon, and right before the demigorgon that band of crazy monsters) i keep a theif weather imoen or yoshimo for traps and locks and i run korgan(amn) sarevok(tob)the first time you beat the game is hard but every import after is almost to easy.mages require to much sleep and to much reloading for when they do die or need to relearn spells, i beat the game through (both amn and tob- with the keep) on the game in what shows 63 days 14 hours (fastest time) (note not actual time but time showing at the bottom of the screen) you dont have to rest as much when you use a regenerating paladin to fight all the fights.ive tried mages ive tried clerics and theifs it just takes way to long. wont touch the monks,fighters,rangers,and druids with a 10 foot pole just to weak and not as good as the others. thats my suggestion for char kit hope it wasnt to boring.......

Facepalms

If you are going to do the re-import trick, know that any character will become insanely powerful, and Monks are some of the most powerful characters.