These are great questions, I started playing the Show 16 about midway through the season on DD (was previously strictly a Franchise and RTTS player). When I started out I had nothing, no knowledge of DD, no players above bronze. I ended up playing around 220 games last year and winning over 150. Considering I started so far behind, I'd venture to say I'm a strong player.
The advice everyone gave you here is good, but don't waste a ton of time playing the market, unless that's something you like to do. You're better off grinding through games and working towards some of the missions (IMO), simply because this will make you a better player then spending all your time on the markets.
I truly don't feel the 'ratings' matter a ton just get players that work well for you. Speed is massively underrated, last year I had Trea Turner hit lead off, he'd basically walk every time and then steal 2nd and 3rd. I was usually up 1 or 2 runs by the end of the first inning. So I'd suggest targeting speed over power. Most people come on wanting to have the big homerun guys, but they're useless if they don't put it out of the park. So I value speed and contact, over power.
Secondly build up your pitching. More then half the people that play this game are terrible hitters and if you have any type of decent pitching, you'll embarrass them. You get into trouble when you have a weak pitcher who starts missing his spots, anything 85+ is going to be able to get you what you need. And don't be afraid to throw balls, you'd be amazed how many players just go up to the plate swinging for the fences. Change ups and curveballs out of the zone will be unhittable for them, but a weak pitcher might hang one of those, that's why I target pitching first.
I bought the odd pack with stubs (usually one a week or so) because I just enjoy the thrill of opening them. Don't let people on here tell you that's dumb, its a game and that's half the fun of it. Sure its not the most economical decision, but it keeps it fun. Last year through random packs I pulled Bettances and Bogarts. They're not incredible, but it certainly was exciting when I opened them.
Finally, target players that work for you. Like I said, I usually build my team around speed and contact with strong pitching because that's the style that suits me best. I'd go up against teams that were entirely diamond players and usually win, because my team worked for my style. These type of players are massively undervalued and usually dirt cheap last year I had Lindor, Bogarts, Odor and Ramos for almost all of the season. They played wonderfully for me because they fit into my style.
That turned into quite the rant, haha. Let me know if you ever want any more advice.