I recommend MyBB or SMF. Both are free, simple to use, and are very easy to administrate. It's definitely easy to install a mod as well. Neither of them need file editing in 99% of cases.
SMF uses their package manager system. When you download a mod (in a zip), all you simply do is upload the zip package in your Admin CP then install the mod. You can also upgrade versions through that as well.
MyBB is pretty simple too. You simply just upload the mod through FTP and activate it.
I recommend both, which are free, but at the end of the day, do what's best for you. I use MyBB on my Rock Band Forum (look on sig). I find it very flexible and great. MyBB's support is a bit better than SMF's.
Well, it depends. Are you willing to spend $150 for a forum software (vBulletin costs a lot more)? I personally own an Invision Power Board license. It costs a lot, but I'm definitely satisfied with it.