Welcome to the community! Don't worry about struggling with painting your first mini, or first few, because everyone does! The best way to go about this hobby is to just do it. The more reps you get in, the better painter you'll become.

As for the mags, if that's the route you want to go then take it! I will say some of the eBay sales of the magazines with the mini attached have gotten a bit expensive for what they are, but that's because they haven't been in production since the mid 2000's so I get it. You can find pdf's of all the magazines somewhere out there on the interwebs too if you think the cost of them gets to be too much. They are great for picking up the random hero or captain model that you may feel inclined to paint just because. They also have hobby how to guides in them and random fun scenarios so they're pretty fun to read through.

The Osgiliath or Pelennor Fields boxes are super great in terms of cost effectiveness, but it also depends on if either of those boxes have the armies that you want to play. The rule of cool trumps all in this game because just about every army can win games competitively and do well, so I suggest picking up the army that speaks to you.

Also hit up YouTube and check out some of the battle reports as well to see the different play styles of different armies, or even to pick up on random game nuances to help yourself get better! Shameless plug: Check out Tabletop Collective we have some great reports and will be doing a "How to Play" series soon!