1. As far as I know, yes, unless you get it used. For shipping from China, it's about $125. For shipping from US (faster), it's about $130.
2. Yes, there is proprietary software but that's only to control the sampling rate, buffer size, and to switch outputs from RCA to headphone out. (You probably won't need to change these settings but I use them to switch from headphones to RCA output speakers.) The actual volume controls can be controlled normally through your computer audio (whether through hotkeys, audio panel, media program, mouse, etc.)
3. I would actually recommend against getting an amp unless you upgrade your headphones. The 555s and 840s don't scale that well with high end amping, ie they are fine with the built in amp on the Musiland.
4. Especially for $100 or less, there will be very little, if none, improvement over sound quality with an amp. Save the money for other hobbies, CDs, future upgrades, girlfriend, etc. 
5. The uDAC is quite nice for audio on the go and it is powered by USB. Most people buy it because 1) it's more publicized and sold by a US company and 2) it's more convenient and smaller (about 1/3 the size of Musiland). That said, the Musiland sounds better, especially without a decent external amp.