Pre-built vs build depends on the value of your time and how much it's a project vs a product. If you build, you need a case (most htpc cases come with power supplies), a motherboard, a cpu, memory, a hard drive, and possibly a wifi adapter (if the motherboard doesn't include it). In general the DIY cases are going to be larger than the pre-built options.
1080p is easy as long as the media uses codecs that you have hardware acceleration for; even ARM can do it for H.264.