Hi! Welcome back to the game.
1) The classes I suggest you look into for soloing capability with potential for party support are:
Pyromancer, Linker, Elementalist, Warlock, and Featherfoot. (You can also choose sage if you really want to, missile hole helps with solo against archer mobs).
Wiz3>Ele3>FF2 or Warlock2 is one of the best builds for damage, and generally, the faster you kill things the easier the game will be. A tip for survivability is to buy Lv15 Condensed HP Potion from market as well as the Huge HP Potions in town Item Merchant (they have separate cooldowns).
If you don’t mind being a bit slower at clearing content then you can also consider:
Sorcerer and Necromancer. Summons boost survivability a lot, and fyi these classes are getting buffs sometime in the near future. Right now, Necromancer is in a better place than Sorcerer, and Necromancer requires less silver.
2) Expenses, potion use, and ease of difficulty are all build dependent. Once you decide on a build, it will be easier to answer this. I don’t think you should have trouble learning to play wizard classes, but there are resources on forum and in game to help you out. Potion use is typically going to be high for all characters (since the rebalance), with the exception of classes that can heal themselves and/or have good invuln/ways of mitigating dmg. For wizard, these classes are Featherfoot, and summoning classes.
3) Different people have fun with different things. If you want to hit hard, then going down the Wizard3 (or Pyro) > Elementalist 3 > Warlock2 or FF2 path sounds right for you.
Leftover skill points are there for your discretion. For Elementalist, the 10 points can be put into freezing sphere or meteor, different people have different preferences.
People also do Pyro with elementalist, but this means you can’t really take warlock (but you can take sage if you want, but that means you won’t be able to get wizard 3 and your casting time will be slower).
5) Just look at the forum post dates to make sure guides are up to date. Can also use a shout megaphone (buy one for one TP from the shop) and ask for help for whatever class you need help for. Usually someone will whisper you!