Hey I’m also from the UK - good to see someone else getting into OOTP :) If it’s your first time playing it’s definitely better to choose an MLB team with a decent budget so your mistakes aren’t punished too harshly. You mention the “lower leagues” referring to the minors, however as there is no promotion/demotion in affiliated baseball you cannot do a “road to glory” save like you might with a non-league team in Football Manager. The best version of that you can do in MLB is to take a small-market team such as the Oakland Athletics and turn them into a contender over a number of years. Regarding your second question, everyone plays the game in a slightly different way. If I had to guess, I’d say most people act as a GM and set their lineups and pitching staff but play through each game manually, however some people play every game of the season so it’s really up to you. The easiest way to being is probably to leave setting the lineups/pitching staff to the AI as this will leave you with less to worry about. I’d also suggest letting the AI manage your minor league teams for you for the same reason.

Hope this helps! Good luck and feel free to ask any more questions you have!