You also wouldn't have the hardest time finding where a 400W+ signal is coming from.

You're going 'up' with a highly directional antenna, and you're going to be transmitting very short bursts. The biggest risk will be the lobes projecting from the antenna some of those will be pointing back at the planet instead of into space. Airtraffic will have the biggest risk of interference (also because the frequency band is close).

So kidding aside you will need to make very certain that (1) you have the right permits and (2) that your gear is not inadvertently sending out a dirty signal. The narrower the better. Better start shopping for a second hand spectrum analyzer ;)

It's not even building a big dish that's the problem, it's that it has to be pointed extremely accurately and stably, and preferably able to track..t

Half of the problem is recieving signals back, as you can't just turn up the power on the satellite. I suppose if you knew exactly what command to send, and there was no form of handshaking this might not be a problem.