My advice would be to check your local Yellow Pages and see if there are any places near you that offer to custom build a desk top PC for you. If there are, this can be a good way to get what you need and not lots of other bits and pieces.
However, there are a few things I'd suggest you try to get on whatever you buy:
-A good fast processor plus plenty of RAM--at least 4 gig.
-Two HDD, one for the OS and software and the other for data, i.e. your tracks. Get disks as fast as you can afford...at least 7200rpm and preferably 10 or 12 thousand rpm.
-Your video card only needs to be mid range for audio use...but it will be very helpful to you to have one that can support two monitors with the output spread between the two, not the same on each.
-USB connections for your interface(s). At least some USB2 and, if possible some USB3 to future proof you.
-A low speed, high volume fan for cooling to keep noises down.
-Since you'll be using an external interface, any built in sound card only needs to be the most basic.
-A good CD drive ...or perhaps 2 for duplication
-OS to match and be compatible with your desired software packages.
Bob