Magento – Learning Magento Plugin Development

There's a lot to unpack in that question, and no easy answer. Also, standard caveats — you're asking a community of people who make (at least part of) their livings developing Magento systems so the answers are going to reflect that.

The way you find a good Magento developer is you stop treating technology in your business as a cost center, and start networking and looking for partners who can help grow your business. You get to know the people who are handling the technology side of your business, you ask them for references from previous clients who are in a similar business to you, you ask your network of business owners similar to yourself who they'd recommend. Like everything in business, it's all about the relationships you build. Magento is a platform you invest in for the future of your business.

As far as a good range for the cost of a PSD to Magento theme — the reason you're having trouble finding a consistant quote for that sort of thing is there's no long term business, even for an independent freelancer, in providing that sort of service. A PSD converted into a standard Magento site with no customizations has no business value, and there's not enough demand for custom e-commerce development for a standard price to emerge from the market. The developers offering you lowball quotes are either inexperienced, using this as a loss leader to sell you additional services later on, a volume based shop that will say yes to anything and then shift the risk onto their contractors, or some combination of all that.

If you feel your business is too small to build the sort of partnership I described above, or build an internal technology department, or you don't have enough experience to manage anonymous freelance resources yourself via freelancer sites (which I'm assuming based on your question) then no, Magento or any self hosted solution is not the right thing for your business. Stick with cloud based providers that handle the technology for you.

Put another way — the days of going online, spending $10,000 and getting a $500,000/year - $1,000,000/year business out of it were a market fluke. Those days are long over.

You've mixed up two concepts here: preferences and plugins.

Preferences are basically rewrites on the class level. In case you define a preference for your controller your preference class will have to extend the original class and you can modify whatever method you want

Declaring preferences is different than declaring plugins you can find details here: http://alanstorm.com/magento_2_object_manager_preferences

Plugins is way different it lets you modify the behavior of a method so it is modifications on the class level. You can see in the official documentation that it has some limitations but it's a powerful feature: http://devdocs.magento.com/guides/v2.0/extension-dev-guide/plugins.html

In your case your di.xml declares a plugin. Plugin classes do not extend the class they're observing though.

You said you extended the controller to get access to the methods. So for the before, around and after methods the first argument is an object that provides access to all the public methods of the observed class.