I have built and supplied machines either ready for Linux (i.e. Some degree of guaranteed compatibility) or pre-installed with Linux so perhaps you could contact me off list with a detail of requirements and I'll see what I can do.
However scratch building machines these days is a game of diminishing returns as it is possible for me to purchase brand name units for pretty much the materials cost of a purpose built machine. The XP licence doesn't really give much of a saving by it's absence as big manufacturers pay nothing like the £60-£75 OEM licence sticker price, in fact I have always believed that REALLY big manufacturers like Dell get it for almost nothing. So really buying a machine without an XP licence is only worthwhile if the Licence has no value to you (i.e. you are sure you are never going to run Windows on that machine).
What you do get with a scratch built machine is an assured upgrade path, in that only "off the shelf" components will be used in it's construction. A lot of the big brand machines now days use custom PSU,s Cases and mainboards which can be somewhat limiting when it comes to upgrades. On the flip side the economics of upgrading any machine these days is suspect, given the price of new complete machines. A CPU upgrade can often lead to a requirement for a different Mainboard and Ram, which pretty much only leaves the Drives and case/PSU....by the time you upgrade the drives are probably too old or too small...you see where I am going with this.
Sort answer, if you feel confident enough with hardware to make a good job of building your own machine then I'd recommend that. In my view it is a worthwhile experience and it puts you in direct control of the hardware inside.
If not and you want the flexibility of a scratch built machine and really don't want an OEM windows license then contact a Linux friendly reseller (myself or there are plenty of others) and get them to build it for you. If you are looking for best bang for the buck then it may be a sad truth that a brand name machine (even with a OEM licence) gives good value.