If you only want 2 channel Audio Ebay is your friend. You can find really high quality stuff for cheap.Is it possible to spend less on the receiver without too much degradation of quality?
What about the Harman Kardon HK 3490?
Also, I can build car subwoofers pretty well, could i possibly save some money and build my own / is there any way to get a power source/crossover plate anywhere?
If you build your own sub you'll need to factor in the cost of an Amp to drive it. Either a plate or pro amp.
For drivers check out the Dayton Reference series. Or you can look around for a car sub that will work in home audio. (JL Audio w7 series)