Somebody mentioned a pelican case. That (or similar eg NEMA 4 or 4x) is a good start. But if you cannot get the heat out of the case it doesn't take much power to cook a PC even at -5C exterior temperature and at 30C it takes very little added power.
It sounds like you are talking hundreds of watts. So you need to make it entirely liquid cooled. This means everything that would normally have a fan -- processor, video card, chipset, and power supply. In addition you will likely need a fan in the enclosure to prevent hot spots, and if that isn't enough you'll need to liquid cool those hot spots.
At this point you should be thinking about submerging all the electronics in an oil tank and circulating the oil thru the radiator as your coolant. (Use light mineral oil, because it will get thick at -5C and you may have to heat it so it will flow thru the lines!) Mineral oil submersion will also protect the electronics against condensation.
The radiator will need to be outside the pelican case. It will most likely need a fan at 30C, so you should use an automotive or similar fan with a proper temperature rating range. Also mount the coolant reservoir externally so you can check the level and fill without opening the enclosure.
If not mineral oil, the cooling system will need to be filled with something to provide freeze protection below your lowest low temperature. (Antifreeze, various alcohols, sugar water, etc.)
The pump will likely need to be controlled to run very slowly when temperatures are cold. But -5C should be okay for everything except perhaps fans and mechanical hard disk (so use SSD). For a less power hungry PC or colder temperatures you might need to insulate the case. If the PC must start when cold -5C is probably okay but you may need to provide auxiliary heating to warm it up to maybe 10C before powering the PC (almost certainly required if using mineral oil or at temperatures colder than -40C, but perhaps at -10C or even at -5C depending on your equipment). You can heat the air in the enclosure but heating the liquid in the cooling system is more efficient if it will thermosiphon within the enclosure. You don't want to pump coolant thru the radiator while trying to heat it.
Mount the equipment without penetrating the case. Keep all electric stuff up as high as possible off the bottom of the case. Condensation or leaks will puddle at the bottom and you want to keep the gear out of that puddle.
Make the cooling, power and connectivity lines come out the bottom of the case (to prevent puddles from slowly seeping in) thru liquid-tite cable glands (to keep out bugs and lightly pressured water). A drop tube around the exit (or each exit) will help protect against high pressure water jets except for a direct stream into the end of the tube. You might also like a valve in the bottom of the case. You can open the valve and if liquid runs out you need to take down and service the system. Do not make any other case penetrations.