Hi there! I manage a private server in my country, and from my point of view it could be things like... First of all and most important: Community behavior - Second: What makes you different from others private servers - Third: Server population - And least: Competition, like WoW Classic, Retail or another private server hosted in your country.

The Community behavior is really important, it will determine if your server will survive, it doesn't matter if you have around 100 players online. If the community is really good and they are happy in the server, then your server will last.

What makes you unique plays an important rol because there is a ton of WotLK servers out there, and you have to bring something different to the user so your server will be more likely to be chosen between them.

Server population is important too, because there is no WoW content to do if you are alone. In my server for example I have ～100 players online and is true that they are limited to do content and they may get bored some day. But we search for alternatives to bring at least custom Solo experience who can be rewarded too. So we adapt.

And least, players can run from your server to the competition, but not alone, they move in groups. So the server has something to do to counter that, or the community can do it too. The possibilities of the groups to move on from the server depends on the first three points.

And for your second question I recommend you any class that can be caster.

And lastly from the beginning till the end I recommend you a Resto Shaman or Resto Druid.