There are a lot of factors in this. I'll try to answer a few...

1.) ease of installation is all dependent on which product you select.

2.) If you are hardwiring, there is no need for a receptacle, as that is not hardwiring. Hardwiring involves going straight from the breaker in the panel to the lighting fixture.

3.) You only need to hardwire each individual lighting fixture if the total load of it meets the load requirements of the breaker you are using. Most likely this will not be so, and you can add them all to the same breaker, and possibly share the common kitchen lighting circuit.

Again, this all depends on which lighting fixture you choose and what their installation recommendations are. I'm thinking you'll want to go with an LED(low-voltage fixture) that will fit nicely under the cabinet. You can either get a hardwired fixture or get a plug-in type fixture. Both will require a transformer which you'll need to hide somewhere. As for the dimmer, usually a regular style dimmer will work, again check with the product recommendations.