Well firstly, welcome back. The game's kind of gone to hell since a damn long time ago, but I still play and enjoy (?) the game to an extent. I'll give you a real quick rundown of what you're asking for. Feel free to post more questions if you get confused.
1. For equipment the only way to stay "competitive" is to either dump a **** ton of time into the game merchanting for meso so you can buy good gear, or dump a metric ton of cash on NX to make your own. For your price range I'd say you could readily purchase a set of level 90 equipment with a bit of %STR on them, which is what you want to aim for. If possible aim for 6% STR per armour item. I'm not too sure on Spears/PAs because I don't play one myself, but I think aiming for a 130+ attack spear is realistic (correct me if I'm wrong) and feasible. Mostly %STR buffs you by a lot. Rings and such are kind of expensive last time I checked (the ones worth getting are generally 100+m each), so you might want to hold off on those for now. As for zHelms, there are new Zak Helmets that are required level 100 called Chaos Zakum Helmets which drop from a harder version of Zakum. There's also Scarlion and Targa helmets which give more stats than zHelms but have a higher level req (level 80). I think they are expensive ATM because the NX:Meso ratio is broken as **** and so SoKs cost a lot.
2. Here's a quick explanation of the potential system: http://www.hidden-street.net/forum/t...L-SYSTEM-GUiDE There are some mistakes in it because it's out dated (New cubes, etc), but it will give you a good idea of what the system is about.
3. Scrolling is still the same as before, except now you can prevent slots from being taken with certain items (White Scroll, Pam's Song, etc) and you can prevent items from breaking from Dark scrolls (or any scroll that breaks upon failure like Potential scrolls and Equipment Enhancement scrolls). If you're talking about just scrolling and not potentialing/"cubing" (see link above for "cubes"), you have to have "0 slots available to upgrade" on an item to use Equipment Enhancement (EE) scrolls. These add random stats to armour (literally anything that's already on there, minus attack), and more attack to weapons. This is also explained in the Potential System Guide I linked to.
4. Most high leveled people will spend a bit of NX at some point, though this obviously has exceptions. As for autopet I would say yes, and also grab auto buff (automatically casts 1 buff all the time) and auto HP/MP potion (automatically uses HP and MP potions when you hit a certain % of your maximum HP/MP). Since Berserk works whenever you're above 30% HP now, it's perfect with Auto HP. Auto HP also tends to bypass potion lag, since it's server-sided.
If you're not ready to more or less dedicate your life to this game or be incredibly bored with it (I chose the second one), I'd recommend you don't bother, unless you have a tight group of friends playing together. There's a lot of great MMORPGs out there that aren't pay-to-win and are much less grindy. Have you considered Guild Wars 2?