That little case they use will be limited by a single hard drive. You need to determine how much space you need and decide will that be enough for you. I think 3TB is the max you can get now in a single drive. But, you could always have an external HD or NAS or raid box.
What I would suggest you do is read reviews from newegg or amazon. Take the parts they use and compare brands. Lot's of reviewers like to list their hardware setup on newegg, and describe how they use the gear. I'm guessing there will be a lot of server/HTPC makers for this type of hardware.
I have personal favorite brands I use when I shop for hardware. I would look at the hardware they've used, if you want to stick to their price limit. Newegg sometimes has barebones systems or motherboard/cpu bundles you can check out. Combo deals too, buy one piece of hardware and get a discount with another piece of hardware.
A75m looks like the MB chipset (I'm not familiar with AMD stuff right now)
A8-3400 is the cpu
They have a mini ITX case. I have my own personal preferences when it comes to cases, and I won't buy the cheap ones anymore, and I only use aluminum cases. Also if you choose mini-itx case, you need a mini-itx format MB. Also some mini cases can only handle 2.5" HD
RAM will be dependent on the MB you choose.
XBL Gamertag- Yrd
PSN - Yerd
Steam - Yrd