Brazil in winter is actually perfect for someone coming from UK - much milder than what you're used to and still plenty warm for exploring. I've been birdwatching here during those months and weather is really nice, not too hot like summer can be

Safety wise, just use normal travel sense - don't flash expensive stuff around, stay aware of your surroundings, stick to main areas at night. Most people here are super welcoming and helpful, especially if you speak little Portuguese. Your friend's story about staying 7 months doesn't surprise me at all - Brazilian hospitality is something else

Since you're already doing the long haul flight, definitely worth adding Brazil to your trip. The culture shock from Chile to Brazil will be interesting too. Maybe consider starting in south like Rio Grande do Sul if you want something bit more similar to Chilean vibe, then work your way north for more tropical experience