I think the first things you should ask yourself is:

If I was to setup a forum, why would people join?

What would people gain from using my forum?

What is my USP?

How will I help pay for my forum?

How will I ensure a regular traffic rate to my forum?

How will I entice people to visit thus justifying my investment?

Can I put up with the constant political bullshit that happens on a forum?

Can I put up with people trying to undermine me at every oppurtunity? As there is always somebody who thinks they know best.

If you don't have answers to that your forum will not succeed so don't even think about investing.

Tim