Don't believe everything you read online. 300 yard shots with an iron sight 454 might not be impossible, but it's definitely irresponsible.
Spend more time practicing than you think you need to, and practice from field positions. Shooting in the field is very different than shooting off a bench.
Keep your shots well within your ability. We have a lot of stands set up for bow hunting that routinely have deer come in to 10-30yd, which is where I started.
Don't start with too much gun. If you're a reloader you can start with whatever you want, but load light. I started with a 454 loaded to 45 Colt levels.
Small game is a great way to practice. I like to spend some time squirrel hunting out of my deer stands prior to deer season. It's a great way to really dial in the mechanics, and at least for me is lower stress than big game hunting. Getting some reps in when buck fever isn't a factor really helps prepare you for when Mr. Big walks by.
There's a handful of handgun field competitions every year, which are a great way to practice too. I shot HHC in Newcastle WY for the first time last year and it's a fantastic match. ABMS outside of Zanesville OH is a great match too.
Use a rest any time you can. There's lots of options that work well. I have a Trifecta rest I got over the summer I plan to use for hunting big ag fields this fall, and a Cole Tac bitty bag that seems to work well out of a stand. Until this year I've been using a game changer style bag, which I think will still be my go to on backpack hunts due to its versatility.
There's not a lot of handgun scopes out there, and most of them suck in one way or another. If you go with a scope beat on hard during the off season. Carry it with you while you're scouting, let it ride around on your tractor or 4 wheeler while you're doing other work, dial up and down an extra turn between targets when you're practicing, stuff like that. If it's going to fail you want it to fail during the off season. Ultradot makes great red dots, and I've never had an issue with them. In my experience Burris's scopes are better than Leupold's.
Like anything, the Indian matters more than the arrow, but make sure you have the best arrow you can get. Most mass produced handguns will need some work done like honing or reaming the cylinders throats to be uniform and a trigger job to get the best performance.