My experience with Deutz engines is with the smallest 4-cylinder engines they make, used in Hesston balers.

Yes, they'll stay cool even in 100F temps, provided you keep the fins clean. You simply must blow any chaff, dust, debris out of the cylinder head fins and off the oil cooling line fins every day. Make sure that you keep the engine oil topped up. If there is ever an oil leak of any kind that makes dust/chaff stick to the fins, fix it ASAP and clean off all the oil residue.

If you do this, they'll run forever.

As for starting in cold weather: ours will start down to about 10F with no block heater, no glow plus and no ether. They're tight engines and they start very, very well.  

If it weren't for the long parts supply chain (Deutz will not sell parts directly to you, you must go through an intermediate dealer, and there are many parts which you can't get anywhere except Deutz), I would be running a Deutz-Allis tractor. But that parts chain gets a lot longer out here in Nevada. Some parts that have to come from Deutz (ie, they're not in Agco/Hesston's inventory) take about eight days to arrive here. That's a long time to have a machine down in a 100-day growing season.