Custom Forum for Website

I'm thinking about adding a forum to my new project, but only if it can blend it seamlessly with the theme of the main site. I recently hired someone to do this for me (not from PeoplePerHour), but after working on 2 different forum platforms, I just had to cut my losses and let the fellow go.

So, just before I give up altogether, I thought I'd ask on here and see what comes of it.

Basically, all I want is this:

Advice on, and installation of, a secure forum platform (I've heard bulletin is good, though a bit pricey). I will obviously purchase a forum if fee-based on the advice of the freelancer.

The forum will then need to be customised as follows:

1. Forum header, including logo, to be the exact same dimensions as the main site so that when visitors go from site to forum there's no possibility of the page header area moving (see attached).

2. Ability to control the forum width (a width of 100% is really not necessary)

3. My own background image to replace the forum background colour

4. And a transparent background behind forums so that the background image is not blocked by solid colour.

See attached an image of how the top of this page might look.

Thanks in advance for any responses to my post.

Andy