Electronic – Where to measure ambient temperature limit inside of an enclosre

Frankly, if you "don't know where to start", get someone that does. It sounds like the cost of failure is very high, so this isn't the time to experiment and learn from mistakes. Find someone that knows this stuff. You can look over their shoulder and try to do as much of the grunt work as possible so next time you're the expert. I think I underestand the basic principles pretty well and could get something working. However, for extra high reliability there is usually much lore and experience that you wouldn't guess from the physics alone. I wouldn't take this on myself without a expert to at least ask occasional questions of and to bless the ultimate design. So from a non-expert view, here are some thoughts:

First 45C isn't all that hot. It's only 113F, which is not out of line for summer shade air temperature in various parts of the world. Ordinary electronics gadgets, unless specifically for lab use or other environmental constraints, will have been designed to run at 45C ambient. Think about it. If you left your cell phone on the dashboard of your car parked in the summer sun in Phoenix, you probably wouldn't blame it for not working. Of course you probably couldn't hold it without hurting yourself either then. But, if you were standing outside under the shade of a tree mid afternoon in summer in Phoenix, you'd probably be pissed of if your cell phone didn't work. That's 113F.

Second, two other advantages you have is that this 45C is highly reliable, and essentially infinite. In other words, it's 45C all the time whether its summer or winter on the surface. Also, you can dump a few kW into a reasonable size "room" of this mine, and it will still be 45C.

So the best solution, as Russell also pointed out, is to spec things so that they run fine at 45C ambient. Space them out enough so each thing sees the 45C ambient, perferably without needing any forced air cooling. That way you don't have to worry about what happens when the coiling breaks down. Again, most off the shelf electronic gear should already be fine with that. Lifetime may go down, so derate. Get good quality stuff that should last 10s of years normally.

If you really need cooling, my first knee jerk reaction is to build a small room and put 2 or 3 air conditioners in it. Make sure any one air conditioner has the capacity to cool the room sufficiently. The multiple air conditioners are for redundancy. You seem to say it's only a few kW, which is small as airconditioners go. 1 kW is only 3400 BTU/hour. Also, it sounds like every last item doesn't need to be cooled, so some of the power you need can be dissipated outside the room and doesn't add to the air conditioner load. 113F isn't that hot and there are certainly off the shelf air conditioners intended to work under those conditions. Having 2 or 3 units always on cooling the room is for reliability. However, this is a area you really should consult an expert on.

There are standardized methods for calculating reliability such as Bellcore Telcordia. Accuracy may be questioned, but they provide a basis for comparison.

I don't imagine there is any need for electrolytic capacitors in an SBC, as the power supply would likely best be separate, and there is little need for tantalum parts with the progress in MLCC caps.

Complex things like memory chips running very hot, and large numbers of MLCC bypass caps should probably figure into your lifetime calculations. Relays have a defined life, optoelectronics generally ages, but VLSI chips also have a temperature-dependent lifetime due to some mechanisms like metal migration. MLCCs can fail shorted.

And, if you're not selling a packaged bulletproof product, the lifetime may be foreshortened by user activities. I once received a "warranty repair" temperature controller that had failed due to a forklift tine penetrating the front panel to a depth of at least 6". They'll do stuff like connect 240VAC to ADC inputs and put electronics in lightning-prone locations.