Do I need to buy a receiver/amp to power my system?

The HK will work great. Yes you need receiver, and yes you can start with a 2.1 setup with pretty much any receiver. Sony is probably the worst of the major name brand receiver manufacturers, but it varies from model to model.

Yes you need a receiver to go between your source devices and your speakers. No - the Harmon Kardon will probably not work. A 2.1 system will work great ... for music. Music is a 2 channel source. Movies and HDTV are 5.1. You "assume" the receiver will let you attach any number of speakers and it will automatically down convert the 5 channels to whatever 1,2,3 or 5 speakers you hook up. This is NOT something most AV Receivers support. Read the fine print on the Harmon receiver to make sure you can run just 2 speakers and a sub with a 5.1 source. The only receivers I know that will let you start with 2 speakers and add more later is the Yamaha RX-V series starting with the RX-V6xx models. I know because I specifically looked for this feature for my bedroom system and have a 2.0 system running right now.

If you want to convert it to 5.1 then i must advice you to go for Sony STR-DN1030. This is a best receiver for your system it costs you around $500, the best part of this receiver is built-in Bluetooth and Wi-Fi, Air Play Support ,video processing and lots more. You can get what you are expecting from this.