That’s a bunch of questions! So fist of all, take it slow. Don’t try to enter the game at high level, the learning process is unique and super fun!
But here some answers:
1) Prepurchase is great to get a large collection. Without it, it‘s allmost impossible to catch up. As you don’t mind spending money, go for it!
At some point, you should decide if you prefere Standard or wild playmode. The later is long term cheaper and more diverse but less balanced and less impacted by new expansions. Also, aquiring those old cards doesn’t come cheap as you can’t open those packs anymore.
2) Classic is allways a safe Investment. If you want to spend 50 bugs on packs on top of prepurchase, those are the way to go.
As for the 2018 expansions, i would mostly ignore them. They are low powerlevel anyways. You may buy single packs with in game Gold though, until you get the first legendary. From april 2020 on, Focus on the new expansions. Until then, there is plenty of other stuff to do!
3) You don’t start at Level 50 because your Account existed before that Update. It is impossible do drop below rank 25, once you reach it. An AI opponent can be found in the Single Player mode, the „innkeeper“, but he is not very challenging ... good for the first days.
4) PVE content has changed a lot over the years. Old content, like Naxxramus, Black rock mountain, league of explorers etc are mini expansions. They cost money but reward a lot of cards that aren‘t found in packs. All of those are in the wild Format though. Ignore them for now, you allways can come back later. Those adventures require you to have a big collection!!
Newer content is free to play but doesn’t reward any cards. Try out Dungeon runs, Monster Hunt and Boom Labs! They are great to learn the game and a ton of fun. It will probably take you some time to complete them all.
The newest adventures cost money again but reward packs aswell. The Price is above the pack value though! It‘s up to you if you want unlock the content. It‘s fun once in a while but all at once is a bit much...
5) Consider crafting whizbang the wonderful. He will be gone in april but he unlocks 18 different and semi powerfull decks at once! (You get a random deck each time you queue ladder)
Finally, have fun! HS is awesome and offers a lot to explore :)