I don't want to be the next Eddy Merckx; Is competitive cycling right for me?
Treating "competitive cyclists" as this single unified group (with three subgrouping) belies some  prejudices.  Like all walks of life there are a diversity of people, all with different motivations, morals and life experiences.  As such there is no single correct answer your various questions.  

For example:


  are people in pelotons generally friendly, supportive, etc. or does
  everyone basically hope the others get hit by a bus so they end up
  winning?


Yes to both.  


  Do people e.g. go for a drink together afterwards, or do they put
  their $10,000 bike in its own personal transport before being
  chauffeured back to their hotels?


Yes to both.

Anecdote for the good in people

I have met many great people in competitive cycling who were very friendly and helpful.  For example, during one long, hot road race a professional rider (who I trained with) waited for me after I got caught in a crash (one of our juniors collided with another member took out our whole team!).  He helped first made sure everyone was ok, then we worked together to get back into the peloton. He didn't need to, and it was a lot of extra work for him to do so as it took us nearly a lap to catch back in.  He still went on to podium that race, but maybe he would have won.  

Anecdote for the bad in people

Once a number of years back when I moved to a new location my first club ride encounter was rather nasty, with a rider threatening to brake check me into the ditch if I drafted him again before the sprint.  It turned out it was the club that was toxic. They even  maligned and mistreated an amazingly friendly and fast former pro from Europe who had recently retired and moved to the area.  Racing pro in Europe is damn hard, the fact he was willing to give his time and energy to the local community was cool, that club had no idea what they were passing up. The ex-pro ended up starting his own club and superseding the toxic club.

Take Home

There is no single unifying experience that makes up competitive cycling.  There are definitely "gun-ho" types.  (It is a competitive sport after all).   Some are abrasive, some are are outgoing and nice, some are reclusive... you get the idea.  Like any competitive sport you will get encounter more Type-A types than you would in regular life.  But that is sort of the point, isn't it?  You are looking for other people to be competitive with.  

My best advice would be to shop around.  If your area has a lot of clubs, do a trial ride with each club to see which one jives with you best.   Some are competitive, some are recreational, some do both.  Some people in the clubs are amazingly friendly and kind, some others are less so.  Also make sure to try and shelve some of your prejudices about "competitive cyclists". Sure some spend too much money on bikes (the bike industry really pushed that hard), but there are many who are quite sensible and still very fast.

Like all of life, you gotta pick and choose.

Finally to answer:


  How "good" should my setup be in order to enjoy such an event and
  easily stay with the pack?


I personally ride a steel bike and have no problems keeping up on fast rides (i.e., 40+ kph). Whether or not you stay with the pack has a lot to do with your base fitness, your max power threshold, how you ride (i.e., positioning in the pack and length and frequency of pulls), how well you anticipate moves within the pack  and the condition of your gear.  If your equipment is not kept up well, doesn't fit, or is highly inefficient then it becomes a hinderance.  Without knowing what you ride this question is too open ended to answer.



PS.  Quitting your job happens when the nice hobby becomes an unhealthy obsession. That said, others would argue you have simply dropped the shackles of societal expectations and become a free spirt.  All I know is that my kids still need to eat.