When I play, I am always rotating soldiers in and out throughout the campaign as they get injured. This ensures a decent stable of "backup" soldiers in place even in the late game. With the faster stat gain, though, if your new guys can survive 5-10 missions, are they able to wear the power armour?
Lets see, noobs have 35-39 strength (weaklings get assigned to radar garrison duties and don't get to wear armor or carry med-kits, high damage weapons and low survival rates). Nano weighs 9kg and power weighs 14kg, med-kits run 2kg, sniper rifles are 6.4 plus a 0.5 clip, a GL is 6kg plus the 1.5 ammo, PB rifle is 3.4 plus the 0.5 clip almost the same as the plasma rifle.
Minimum, without reloads or smokes or anything else will be about 15kg with nano-armor (everyone can do this but nano isn't cutting it any more by this time) and almost 20 with power armor. So we need a 40 strength score for the least equipped rifle man in power armor, 46 for a sniper and 47 for an exploder.
Looking at the stats I posted it will take 10 to 20 missions for a soldier to be capable of wearing power armor. If you want ammo reloads, IR goggles, and grenades it's definitely going to be 20 missions. Given the alien preference for shooting the noobs first (lowest health targeting priority) trying to build these guys up late-game is just not happening for me. It's really uncanny how often the new guy on the team gets blasted when the aliens have a choice of targets.
As for the randomness of UFOs appearing, is it always a fixed chance or does it modulate based on recent activity? For example is it something like a fixed 10% chance per day or is it closer to 10% - 1% per UFO/landing in the last three days?