Temp rang is just that, It will move X amount of BTU Inside that range at Y super heat
You are correct as far as lower temp = lower capacity, thats what the valve does, it throttles the refrigerant flow to match evap load to maintain super heat. Its most effective range is inside the window they list 15F to -40F is a common commercial refrigeration range.
You are correct again, a receiver acts as a buffer tank to ensure a solid liquid column to the valve, as the valve throttles back the now un-needed flow is held in the receiver until called upon by the TXV.
You are correct as to the load, The BTU/H is what is needed strictly to hold the 400w load, how ever we all so have to A:pull it down to that, B:We have to fight external heat influx, C: We have the heat of the compressor too we must reject.
Your average system of 6,000BTU will use the 6,000 mostly to pull down the loaded area, then as it throttles to temp in the end it may only be moving 1,000BTU once there to maintain the temp.
As for the valve ranges, it is the max the valve can move wide open at its rated refrigerant at its rated pressure drop during pull down.
Simple calcs or selection in refrigeration? Not really, it just seems simple after a while as you know what to ruffly expect, but you get ruff performance to go along with the guesstimate.
For a good performing chiller with allot of room for expansion I'd say find a nice 3,100BTU @ 45F evap compressor, with a 5,000BTU condenser and a 1/4Ton mid temp TXV, and use a 6,000BTU exchanger, the ones RunMC has are nice and cheap (Good deal for us in Canada! even after shipping!!) for that as an evap you wouldn't need an EQ valve.
That should offer a good performing system with low power consumption with room to expand and should hold you at a nice chilly load.
Best way is get one to play with, they are extremely forgiving about charge and system construction, as you gain experience with how they preform and such you can focuser on streamlining and improving your design.
Check out this:
http://hvacwebtech.com/marksM2.htm < TXVs explained.
