Diesel cold starting

You should be fine on almost any diesel in good condition without a cold start aid down to around 40 degrees.  Below that, most diesels have some sort of starting aid, such as a thermostart or some other type of intake manifold heater or glow plugs which will get you fired up in freezing conditions.  The perkins in the 175 is supposed to be a really good cold weather starter for a diesel from what I have heard.  But if it's worn out, it doesn't matter what kind of engine you have, it won't want to start in the cold.  Also, generally you will find that non-turbo tractors have a higher compression ration than their turbo counterparts, so that can be something to look for in a tractor that will start easily in cold weather.  For example, naturally aspirated 301s had 16.3:1 compression, whereas all but the earliest turbo 301s were dropped to 15:1 (the 6080 with the 2/3s of a 301, 200 cu in motor, had 14:1 compression if I remember correctly from what I was told due to high boost pressures).