You do what you can to train for the specific races. In general, the better trained/fitter you are overall, the better you will cope regardless of the race specific specialization. There are some ultra runners that live in flat areas that do OK in mountain ultras for example. Some things I have heard people do is run up and down multi-story car parks at night time, or the fire-escape stair cases - works for people that live in cities. Weighted vests and lots of hill repeats on whatever is the steepest hill you can find. I am still amazed how well some people do living in flatter places cope. I raced this weekend, a wicked hard mountain race (110km, 8700m climbing) and spent a few hours with a Dutch guy. I get in plenty of vertical a week, but it is on shorter hills and not so technical and I find this limiting. There is a hill behind my house abut 250-400m, which I go up and down on as much as possible. But nothing crushes your legs like 1600m downhill, or can compare to the mental state when you are at the bottom of a 2000m climb.
For heat, you do adapt relatively quickly but not in time for a short holiday. Obviously you can try and do your training in the hottest part of the day. For races like Badwater 135 in Death Valley, people train with a treadmill inside with the heat up full blast.
For support, I never have support. It would obviously be nice but is pretty selfish thing IMO. At this weekend's race I saw support crew coming out at 4am start, and every few hours, midnight, 4am, 6am.... If you are a professional sure, but I don;t see the need as an amateur. Everything you need to finish is provided at the race so there is never a big problem, eit is for comfort. Some things I wanted would be some better food, the envy when i see someones support hand out a big pizza at 2am after 22 hours of running with basic aid station food....
Sorry, no real answers.