Build your storage into a corner of the basement. Preferably have the longest wall run along an outside foundation wall.
Frame as you would for a typical closet. The interior walls s/b covered with an exterior grade sheathing material that has a moisture barrier (Georgia Pacific DensElement™ Barrier System) or bathroom green board. Remember, the interior of the walk in is like the exterior of the house - you are keeping moisture from leaving your room the same as keeping moisture from entering your house... The exterior of the walk in gets regular sheet rock and can be painted/finished like the rest of the basement.
Use an exterior half glass pane patio door = $400....not a closet door. Door should open outward - and the door face that would normally be on the outside of the house is inside the walk in.
Interior walls and ceiling get covered with 1/4" veneer 4'x8' spanish cedar. Cost from Bob is $72/sheet but it really increases shipping $$ - you can get locally. If your room will be 4'x8' (32 sq feet) you'll need 8 boards to cover all walls, around the door and ceiling.
You will need solid spanish cedar trim, casing, base molding and inside/outside corner covers to seal all the seams from Bob.
3 x 96" door/window casing @ $32/piece
3 x 96" base molding @ $32/piece
2 x 96" seam cover @ $14/piece
8 x 96" inside corner covers @ $14/piece
You can put shelves along the long 8' wall and stainless steel racks along the side walls. Solid spanish cedar shelves are 12" wide with a 2" lip. It's 5 pieces x 8' @ $100/shelf.
15 x Adjustable angle brackets/shelves @ $3.50/piece and 3 shelf holders @ $13/piece.
Total was $1500 plus shipping for a MONSTER 4' x 8' room.
Add the cost of the door, the sheet rock, framing materials, flooring and some lighting/electrical for outlet plus +/- $1000 - $1500 for labor...depends where you live and how handy you are...
I was going to use a ultrasonic cool mist humidifier tied into a Spyder Robotics Herpstat 2 Temperature and humidity controller (for reptiles) to report temps and turn on/off the humidifier and control a few aux fans for circulation. Cost of this part about $300.
All in probably $5000. That's more space than you will ever need. Just the 2 side shelves are each 36" wide x 12" deep x 7' high. That's the equiv of a large tower humi...each rack! Plus you have the 8' shelves to display all the open boxes on an angle...