There are probably computer shops near you who would be delighted to assemble a system for you. Here's my starting point for a pretty beefy server (US $):
i5 4590 quad-core processor $200
ASUS Z87-A mid-range motherboard $125
16 GB DDR3 $175
2x1TB WD "black" hard drives in RAID 1 $170
Antec Sonata case $70
Antec EA-650 power supply $90 (Don't go cheap on the power supply!)
Optical drive $20

Oops, I went past the $700 target, it's an $850 configuration.

As you add web sites, you might want to add memory, and that's possible. If you run out of processing power, add another computer and split the web sites over the two computers. By starting with "best of breed" components, the computer should last a long time. Just make sure you run it in a dust-free environment; when the CPU heat sink fills with dust, it becomes a big problem.