Polar M430 is a good choice, but I'd also look into the Garmin Venu Sq. It's around $180 and has all the basics plus some nice features like music storage and playback. Worth checking out if you want a bit more bang for your buck.
Good for Madison de Rozario, but let's not forget, it's a wheelchair event. I'm not taking anything away from her achievement, but it's not the same as running on two legs. Still, congrats to her and the other winners.
I've had my share of GPS issues too. I've learned to take it with a grain of salt. It's not like I'm trying to win the Olympics here. As long as it's close, I'm good. If I'm really concerned, I'll map it out myself. It's not worth stressing over.
200m repeats are a good starting point, but don't overdo it. Focus on form and pacing, and don't be too proud to walk if you need to.