communities. the button in the bottom right corner (or shift+o) brings up the communities menu, where you can find people. you can look for groups for powerleveling, bountyfarming, regions/countries - even reddit.
at max level, depending on your money even later. As you said, it's not practical to transmog everything, so most people wait untill they have the items they want/need for the build before they go into cosmetics. If you feel that you have too much gold, no one is stopping you though.
Adventure mode: I'm going to put this below, since it's going to be lengthy.
There are a few wikis (Diablofans wiki for example, and the official site has a bit of lore as well: http://eu.battle.net/d3/en/game/ .
The combat is rather simple mechanical wise, most of it comes down to positioning and timing a few key skills correctly.
5b: Endgame content: Once you are done with the Story, and have killed many bosses, Diablo of course being one of them, you should look into the mentioned adventure mode.
Adventure mode: Starting a game in this mode unlocks the whole map - all 5 acts, all waypoints. Each act will offer 5 special tasks (bounties) - each of which will reward you with a bit of gold and experience. Completing all 5 bounties of one act will give you further rewards in the form of a goody-bag, containing blood shards, a few items and crafting materials, as well as act-specific legendaries only obtainable from these bounty-bags.
Adventure mode also gives you acces to rifts.
Rifts are randomly generated levels from all acts, populated with random mobs (again: all acts). Killing these fills a progress bar (elite and champs drop special globes that help fill it faster). When the bar is filled, a rift guardian (big boss) appears, killing him grants blood shards, items and possibly a greater rift key.
Rifts also have a higher magic find by default, they are a good way to get geared.
Greater Rifts are similar to rifts, with a few differences: They are timed (15 minutes), and they do not offer any loot or gold while progressing through them. The Greater rift guardian will drop more to compensate. If you manage to finish the rift guardian within the allotted 15 minutes gives you the chance to upgrade special legendary gems (which also drop only from greater rift guardians).
There is a lot, lot more to adventure mode, but I'd recommend asking the specific questions as you get them, as this post will get too long and winded otherwise.