If it were me, going off those pictures and assuming that the hole in the wall in the second picture is the chimney among other things. I would ,.........
1. Wrap and seal the the A/C pipe.
1 gal elastomeric paint $30
5 rolls of 10mil pipe wrap $30
Paint wrap then paint again. Left undisturbed you never have to worry about it again.
2. I would put in a floor and frame it with steel studs. For the decking use pressure treated 3/4" plywood. Then vapor seal the ceiling with visqueen. Next build the walls this is where the steel studs are great you can go 24" center's on non-load bearing walls and they are mold, mildew, and terminite resistant. Use standard framing practices for doorways.
Studs $3-$5 per 10ft.
Track $4-$6 per 10ft.
Plywood $40 per 4x8
Visqueen $20
3. Electrical. Since we are using steel studs you have to use m/c (armor cable). If you shop around you can find 12/2 mclite for $15 for 25ft. Don't know how far or what amp service you have. Personally I would run 3 circuits.
Lighting: two boxes on the ceiling about a foot from the short walls and two on the long walls about a foot down from the ceiling. Use 4" boxes and 2 device mud rings. One circut (outlet/device) will be for lighting and one for fans. For the lights I would use this timer
https://www.amazon.com/gp/aw/d/B0000...mer&th=1&psc=1
They are user friendly easy to program and in 30years I've only seen one fail and it was over 70years old.
Fans: okay is that a double flue chimney? Was there a coal furnace there? If so I'd drop a fan circuit there and including the ones in the ceiling I would put one on each short wall with a pump circuit.
Pump/Auxillary: single outlets center line of long wall 20" up from the deck. Short Walls double outlet one circuit fan and one pump/auxiliary center line 20" from the deck.
Coverings: use green board instead of regular drywall and paint with the cheapest semigloss white paint you can find. VCT for the floor.
​​​​​​​All that a door and BOOM! There you have it a Commercial Grade grow room.
(There's a few things I missed but I just realized how long it took me to write this and am now sleepyhead)
​