Stupidly high-altitude drone

This is an idea that has been sitting for a while gathering dust at the back of the broom closet I use as a brain these days. To be honest, I'm posting it in the hope of getting some useful feedback, even though this is the halfbakery.

The simple aim is to make a model airplane capable of reaching stupidly high altitudes. It will be electrically powered, using onboard batteries at low altitude, and then jettisoning these and relying on solar panels alone for the latter part of the flight.

The problems of reaching stupidly high altitudes are, it seems to me, two-fold. First, the air is obviously stupidly thin, meaning that it provides very little lift. However, it also provides very little drag, meaning that you should be able to fly stupidly fast (for a given amount of thrust). I'm hoping that the lift:drag ratio (which determines flyability) will remain OK even up to very great altitudes.

The second problem is propulsion. A propellor, or even a ducted fan, is fine at low altitude. But, at very high altitude, I suspect it would have to spin impossibly fast to generate useful thrust. So, our model plane will carry an electric motor and propellor, but then jettison these to reduce weight once it gets too high (and fast) for them to be useful.

However, it is possible to get modest amounts of thrust using electrostatics. Google "electrostatic lifter" and you'll see plenty of videos. The basic idea is that you put a huge voltage between a leading-edge electrode and a trailing-edge wire. Ions flow between the two, and (and this is what gives the thrust) entrain un-ionized air molecules. You can produce about 5 grams of thrust per watt of electricity using this system.

(Please don't tell me that any thrust produced by the ions will be nullified when they hit the trailing electrode. I know. But the point is that uncharged air is entrained by the ions as they move between the two electrodes, and it's this uncharged air that is accelerated overall, providing thrust.)

Now, is this method of propulsion any use to us? Possibly. The ions themselves are accelerated to very high velocities. This means that they can potentially accelerate entrained air up to very high velocities. At normal atmospheric pressure, the air is quite dense and hence the entrainment only accelerates it by a limited amount. But, at very low pressure, I suspect that entrained air can be accelerated to a much greater velocity (relative to the thruster). On the other hand, there is much less air to be entrained in the first place. But on yet another hand, if the forward velocity of the thruster is high enough, the ion train will encounter more air molecules. What I don't know is how these various factors balance out.

That, basically, is it. A simple electric model airplane, but using electrostatic thrusters to reach far, far into the uppermost upper atmosphere. What would limit the altitude?