Hiya Silversurfer! Did a similar exercise about a year ago building my toolkit, but with slightly different emphasis. Just been looking at Sliders too and the two I've recommended are both responsive. [Slider Pro and [URL="http://soliloquywp.com/"]Soliloquy](http://sliderpro.net/). I've used Slider Pro but just had Soliloquy recommended by another plugin developer I trust this morning.
I tend to use Code Canyon as a good supplier of plugins for my toolkit. Although the license doesn't require them to support their plugins, if you stick to the trusted providers or read the ratings/comments you can easily sort out which developers are worth their salt. Prices are good and for the most part a little research goes a long way for the small prices they charge. There are other libraries but I find it easy to shift through and find quality at CC.
Ecommerce is a little trickier but thankfully there is some good competition going on. I've recently worked with Woocommerce, it has had a pretty aggressive development cycle at the moment. They're the new kid on the block and they want to prove themselves. Being in more recent development I'd certainly give them a look in. They're bound to have had to tackle the responsive design issues.
There are a ton of social networking plugins and options. From what I've seen lots of people are implementing these well and you are spoilt for choice.
Hope the helps.