Sorry about the curtness of the previous post, it was done in a rush.
A few extra points:
If you specify your problems into the appropriate forums you might receive better and more expert response, as you have many different queries.
You say you are currently using the garage as a gym, so I assume that apart from the temperature it has been acceptable? If so I would concentrate my efforts on heating, ventilation and air conditioning.
Heating: Your 400 watts of heating is absolute minimum, I would suspect that it will still require about 1 hour to raise the temperature of the air alone by a few degrees, not considering heat loss to all objects in the room and the roof/ceiling, walls and floor. I guess the
objects in the room are going to 'soak up' the heat. You ought to consider putting the heating on a time clock to come on well before you wake up, or even leaving it on at night on a thermostat.
Ventilation: I mentioned before about considering forced, controllable ventilation. Natural ventilation often relies on air pressure difference generated by prevailing winds. Often no wind means far less ventilation. You'll need to increase the ventilation while you're training due to higher humidity (& maybe need to raise the temperature of the incoming air). Similarly you'll want to decrease the ventilation overnight to conserve heat and give the heating a chance in the mornings. Dehumidifier might help with humidity but won't remove odours
Air conditioning: I assume that you have a similar but opposite problem in the summer. If you do install air conditioning you'll want to reduce the ventilation. Air conditioning will also act as a dehumidifier.
Floor - 50mm thick board. Not sure what material you'll find this thickness. You could laminate with 2X25mm plyboard. But you'll only need this thickness in areas that weights may be dropped. Normally one layer of 25mm plyboard will be adequate. Normal floors are chipboard which is weaker and more prone to failure due to damp. Again ask for advice on the other forums. I would think you'd need to lay the upper layer at right angles to the lower. Again ask in Gym-specific forums assuming there are such things on the 'net. What I said about insulation especially applies to floors in that payback period is perhaps longest on floor insulation.
So consider not insulating the floor and laying one layer of plyboard direct on the concrete floor over the top of the dpm. This will then provide a sacrificial floor if necessary. You'll need to consider the problem of laying the dpm direct onto the concrete because it will suffer from any defects in the concrete surface. May be better to consider a painted dpm on the surface or laying some form of cushionning material that is not subject to damp, in order to preserve the life of the dpm. Alternatively use a marine/exterior grade plyboard which will withstand moisture and therefore no dpm needed.
Fixing battens to walls may not be necesary. You can fix a 'sole plate' to the floor and a 'header' to the ceiling joists. Studwork on the wall will then be fixed between these.
Your wall finish is a cost/aesthetics/performance/workability choice. Plasterboard perhaps is the best aesthetic choice and maybe the best performance. Again ask in the other forums.
To summarise I would prioritise:
Heating with appropriate controls
Ventilation with appropriate controls & possibly dehumidifier
Air conditioning can be considered at a later date but plan for it now.
Ceiling and insulation. How's your headheight? Consider insulation direct to under rafters so as to maintain headheight. Then you don't need a ceiling.
Garage Door!
Floor covering as discussed above.
Walls.
This can also allow a phased schedule over a period of time.
Finally, but not least, one other consideration: Single skin blockwork exterior finish. Is it porous? If so you need to address this issue.
