we would like to create a multiplayer online virtual world.
We would like our users to be able to:
- visit the world
- see, meet and chat with other users
- get information (coming from external sources)
- pay for things purchased in the world
2.5D
I'm writing the business plan and I would like expert advice on the big questions:
1- how long would it take to develop?
2- can we reuse existing stuff? (I've browsed through dozens of game engines trying to understand what they do, and if they can match our needs, but it's very hard for a noob like me!)
3- how many people to do the job?
4- which kind of people (skills, experience) do we need to hire?
5- alternately, would it be a good idea to work with a game design company?
6- What would then be the best candidates?

1. A minimum of two years to get a minimum acceptable virtual world - and then everybody continues working forever, as long as your virtual world runs.
2. Only what you buy. All games involve creating new code, even if someone else has done a similar feature. The engine is never solely sufficient.
3. More than 30.
4. All kinds: especially programmers. You need designers, artists, producers, testers - and since it's a virtual world, they'll be constantly working even after the 2 years.
5. You mean a game developer? Absolutely, you want to hire a company that already has experience, rather than trying to create a new company as a novice to the field.
6. You have to do research.

Oh, wait. You also asked "How difficult."

"Very."

Oh, wait. In your subject line you also asked "How much."

The amount is measured in millions.

1. but time is more critical for us: we need to deliver quickly.
2. visiting the world will not be the initial motivation for people so if the world is very small at the beginning that is not a problem.
3. Knowing that it would take 2 years and 30 people to complete a full-featured world,
4. I'd like to get an understanding of the timeline, and evolution of the roles of people in the team:

1. Then it is doomed to fail. If you try to rush this, it will turn out badly.

2. Perhaps so, but you want (as you said before) a "virtual world" that's scalable and involves a payment system or in-game money or some such. Whether this is the central experience or not, it's not a small task. You have to build the entire infrastructure now, with all the scalability hooks now, with the payment/money system now, the analytics system now, the chat system now, the servers, backups, and migration system now. You're building one finished room, but that room doesn't float in the air. It has to exist within a full framework, underneath a finished roof, with all the outside walls and windows and doors, plumbing, electrical, gas, cable, sidewalk, porch, and hallways/stairways leading to that finished room. When you get into that finished room, you won't care that the living room, dining room, and kitchen aren't ready yet, but you will find that you can't live in that room without also having a bathroom that's finished too.

3. You don't know that it will take 2 years and 30 people. Some stranger on the internet, who knows nothing about your project, gave you those numbers out of his hat after you asked a question on a public forum. You need to do your own research, consulting with NDA'd professionals with whom you share full details of what you're trying to do, and what constitutes a minimally releasable user experience. It might take less than 2 years, but to do that might mean it will need more people. It might not. These numbers (2 years, 30 people) are worth what you paid for them: nothing.

4. Then you need to start spending time and money to find that out. Asking for free answers on the internet is not the way to go about it. Get professional.