Verbosh is one of my favorite Judges Guild modules. It is really a sandbox campaign in one booklet. There are lots of different adventures in the module, including adventures of different types (dungeon, tower, underwater, overland, city, undercity, ruined city). The book is just packed with things to do and the minimal AD&D stats given make room for many many things.
My advice would be to begin your campaign outside the gates of Verbosh, with each player holding their starting gold in hand.
Make the characters shop in Verbosh to get their equipment. The shops are described, and clever players can gain advantages. This was great fun when we played it back in high school.
This can be especially fun if the gate guard convinces one or more of the PC's to exchange their gold for worthless Verboshian paper currency.
Allow the player characters to pick up the various rumors and such as part of their gaming experience and let the sandbox fun begin. One complication might be that inexperienced players may search out challenges they should not try. That might or might not be OK depending on your particular
DM style. A good
DM should be able to steer the action.
Have fun with the rat with 26 hit points!
Caverns of Thracia can be placed in the Verbosh outdoor setting anywhere you want. It would certainly fit in any of the mountains nearby. In my opinion, the AD&D version of
Cavens of Thracia is well-written, but has some elements that are difficult to
DM or even explain to the party. (I haven't yet read my copy of the 3.5 version). When I ran it a few years ago, I modified some of the content to suit my own needs. Still, I found
Caverns of Thracia OK to
DM and I think you could easily make it work with
Verbosh.
Verbosh is a humorous read. In my opinion, it is the most old school of the Judges Guild products aside from
Dark Tower. (And
Dark Tower would also make a nice addition to a Verbosh campaign! The pdf is available for cheap.) There are clever puns, hilarious situations, funny backstory and the like hidden all over
Verbosh. A good
DM can turn
Verbosh into quite a tale. My
DM was a good role-player and some of the Verboshian NPC's were very memorable.
I'm guessing you plan to run
Verbosh as an AD&D setting. If so, there's no reason every character class could not fit. If you're planning to run it as 3.0 or higher, my advice is to omit druids (because they are impossibly stupid in that edition). If you modify the setting to
Pathfinder, my advice is to limit the number of spell-casters.