Just looked at the Red Dot units.  They look like a nice drop-in addition, especially for an already sealed cab.  The 33 amp current draw seems quite steep, but a few bucks for alternator maintenance is a good deal for the extra working time and energy of the operators if it's really hot or cold.

 I've got a 454, 484, 584, and 885 with cabs of one sort or another.  They're all straddlemount tractors with aftermarket cabs with heat, no air.  All but the 454 have air options available from the manufacturer (we cobbled together a cab around the rops for that one ourselves, but that's a PITA doing it and living with it afterwards).  It'd be worth a phonecall to Laurin, Sims, Year-A-Round or whatever to see what they offer for that cab and tractor before going with a universal all electrical option.  We just either take the doors off or tie them open (easier but hard on hinges) in the summer here.  If you've got no heat and don't need a lot, or want something additional to keep your rear window defrosted, just get an automotive 12v heater (plugs into the cig lighter) from your local autoparts place and blow it where you want it on your glass or feet.

 Good heat is the easiest to add if your cab has the overhead bulge to hold the fan, heatercore, and switches.  Mount the heatercore, pick a fan of your choice (12v schoolbus dash fans work OK, but a junkyard squirrelcage fan would be good too), wire it, and you're good to go.  x84 series IH tractors have troubles pushing coolant up into an overhead heater core though, so if you think you've done everything right and you're still getting no flow, it's not your fault.  Put a 12v pump inline and flick a switch when you're cold.

 Biggest problem I have with straddlemount cabs is sealing them.  If they aren't sealed well your feet'll freeze in the winter even with good overhead heat and the AC will be given a huge workout in the summer.