If you are looking for good IR cameras, then go with either the ThermoVision Security HD or the WideEye model (both made by FLIR Systems). If you are looking for state of the art spy cameras, then search for stuff like the pinhole-optical camera.
Those are thermal imaging cameras, not IR light sensitive cameras. They start at about $6000 for a 160x120 resolution camera... Not quite in this guys price range!
Back to the op:
Its cheaper to use fixed position dome cameras than PTZ cameras if all you are doing is recording - i.e. nobody sitting there controlling the cameras. With modern megapixel cctv cameras you can cover a wider angle of view than with a vga res camera and digitally enlarge the area you want an image from. Also use wired cameras with PoE, dont bother with wireless as you'll have the hassle of getting power to them, whereas running a CAT5 cable is usually a lot easier.
Megapixel cctv cameras start at 1MP and go right up to 10MP, although expect to pay a lot for anything above 2MP (1600x1200). There are specific cameras for different situtations - megapixel cameras usually work best in daylight or artificial visible light, wide dynamic range cameras work well in low light situations, and CCD sensor based (as opposed to CMOS sensor - which are usually megapixel cameras) cameras work best under IR illumination. Mobotix get around this buy using a daytime colour camera and a nightime b/w IR sensitive camera in some of their units (M12 is one of them), but one of those cameras is pretty much your budget!
Oh and dont buy an 'in the box bundle' they are usually total crap.
If you intend to use non-visible light illumination then check the cameras spec against the illuminators spec - there are different IR wavelengths used and cameras are usually more sensitive to a specific wavelength range.
Edit: Check out the Vivotek camera range, they are quite reasonably priced, and having used them myself I can vouch for their quality. Axis and Mobotix are very good cameras, but they come at a price - particularly so with the Mobotix cameras.
For recording have a look at the QNAP Viostor NVR units - they are specifically designed for cctv with a feature set to suit, also if you find those too expensive QNAPs regular nas boxes will support upto 4 IP cameras with a more basic feature set (lack of event triggers and motion detection afaik). The other (cheaper!) option is to find some open source cctv dvr software and run it on a dedicated box, i think these are usually linux based but i cant say ive ever played about with them, ive always used Viostors. Dont forget a PoE switch if you use PoE cameras!
http://www.arecontvision.com/index.php?section=gallery&s_fk_category_id=8
The Arecont website has a gallery of snapshots from its cctv cameras at 1.3, 2, 3, 5 and 8MP resolutions which if you download the image then view it on your pc will give you some idea of the resolution - and the scope for zooming in afterwards from the various resolution cameras.