There are truck loads of HTPC threads, please do search and I am sure most of the questions you have in mind will be answered by me or somebody else ..... the latest HOT HTPC thread is
http://www.hifivision.com/home-theater-pc-htpc-media-pc/62385-yet-another-htpc-advice-thread.html
go through it and you should be on the right track ...... if there is any very specific query shoot here and I shall try to answer that ....
BTW HTPC is just a PC tuned a bit for Home Theatre requirements. I am not sure where r u located but from where ever you will purchase the hardware, same people will also help you assemble for a very nominal price.
There are branded PCs as well but are a bit expensive, then there are APU based PC like E350N where we have GPU and Proc all on one chip. You still need RAM and HDD, rest is same as any other assembled PC. Other type these days is SFF-PCs (small form factor PCs like NUC and BRIX), which are SOC type assembly. It not necessary you need to install RAM and HDD, I mean yes barebone ones you need to, but you can get complete package as well with everything install and ready, you just need to connect the display, put the OS and use it as you want to. (
Intel NUC DN2820FYKH Celeron N2820 2 4GHz DC 4GB DDR3 1 TB HDD Wifi BT Hdmi | eBay)
Last and current gen latest type are Compute Stick which are PC on a HDMI stick, recent iterations have become pretty powerful, (eg Intel PPSTCK1A32WFC Bay Trail-T Compute Stick ) however feasibility as an HTPC is yet to be determined, with latest Intel graphics drivers for Bay Trail-T (32-bit) we have support for bitstreaming of DTS, Dolby Digital and Dolby Digital Plus. This means that Netflix and other similar OTT sites are in the clear, but not HD audio with BD rips or ISO, also with only wifi n these are anyhow not recomended ......