Web 2.0s

Is building web 2.0s with unique articles good enough to rank a keyword with low competition? Should I build links to those?

Yes, but what you consider a low competition keyword may not necessarily be so. Build web2.0s as layer 1 and power them up with layer 2 links.

Depends on strength of top ranking sites. Yes, you can build links to your web 2.0.

Simple answer, yes. If you take a look through many of the SEO - link packages sold on BHW many include web 2.0's built with a variety of content related to the niche the money site is looking to target.
If you search through, sadly I cannot remember the link, although there is a guide to building a set of web 2.0's, then pointing to the money site, and improving rankings. Also they are a great method of pushing links towards your website with a safety net, ie if they receive a manual action web span notification from google, simply remove the links from the web 2.0 and little / no damage is done to the core money site.

add images / videos and links to authority sites not just your own

Its good to take start with web 2.0s which must be submit with unique content, pictures and videos. But you may have to build other links for diversification and keyword variation. If you make a link mixture then your website rank soon on Google and take a long stay on first page.

The quality of your web2.0s would matter here as you can see from the various recommendations. Try it out instead of asking and going by what you are told to do. Once you see some success, you will be able to do the same with additional keywords.