Hi! Thanks for showing interest in my game, there's plenty of people in the reddit community on /r/paydaytheheist that will happily answer any of your questions in the future!
As for the questions in your post: I've dumped 945 hours according to steam, which probably translates to ~900 hours of actual gameplay. I find the gunplay fun, and the heists varied enough to keep my interest over time, but the best thing that's helped keep my own interest in the game are the people I play with regularly. I play in public matches every so often but my close gaming friends make the most magical moments.
To your concerns: the earliest levels come really fast, slowing down at around level 70 or so, and there's a secondary prestige-like system called "Infamy" the first 5 levels of which reduce the point requirement to reach higher skills so getting a good build that reaches high into a tree is pretty simple.
Their DLC system is set up so that very few are vital. You can freely join but not host any missions that you don't own, so you're free to skip over those unless they have something you want. I believe the current community consensus is such:
If you have the patience, ALWAYS wait for a sale, prices drop down greatly during this time, and mods in general order of importance are as follows
you NEED the Gage Mod Courier Pack. It lets you pick up packages spread around every map to receive free mods for most guns. Free, as in "costs nothing to put on or take off" and it makes certain guns like the Car-4 much more usable, and if you like that or the AK family get the AK/CAR mod pack to make those guns even better.
you should PROBABLY have: The Gage Historical pack which gives you a sniper rifle, and LMG, and a solid SMG (the former 2 are generally DLC-only weapons). if you already know you'll love those then consider grabbing the Gage Sniper Pack or the Gage Weapon Pack #2. Also consider the BBQ pack or Gage weapon Pack #1 to have grenades (Molotovs and Frags, respectively.)
If you want to deal with shields easier, consder the Gage Sniper Pack (or historical pack's sniper) or the Gage Shotgun pack (especially if you like shotguns) as snipers all pierce the shield, and the HE rounds from the sniper pack stuns them. Grenades also work fine for this if you've picked on up
*look at the perk decks in the skill menu, most of the default ones are solid by themselves, but if you like a certain way of playing then character packs and their respective perk deck are generally better for more focused play (ie. stealth only via Clover, or Loud Only via Dragen or Jacket) so pick one or two of them up if you have money
*Beyond that, anything that looks fun. Melee weapons have a serious power creep so the newest DLC tends to have the best ones, but there are notable exceptions. the Spear of Freedom from the historical pack has great reach and damage, Clover's pack comes with a Shillelagh which has some of the best knockdown for it's long reach, and the Gage Shotgun Pack has a Telescopic Baton which has the highest range for max concealment, great for stealth missions. none of this is super important, however.