Don't invest in analogue system in second decade of 21st century. There are only two reasons they are still manufactured - simplicity of tech and price. So yes, it's cheap but it's like buying into VHS in BluRay times. It's cheap for a reason. If you ever had a contact with any modern recording equipment you will be disappointed with the quality, regardless of TVL and once you invest in DVR for your BNC cameras you'll end up spending and improving cameras and picking sensors, all in a tech that simply cannot be improved for no money.
Max analogue CCTV resolution - D1 (704x576) even when coupled with cameras claiming best TVL values, let's say 700, is all relatively meaningless, because a) PAL signal maxes out at 625 lines with overheads and in closed circuit systems is delivered at 575 horizontal lines per picture frame or 287.5 per field, b) by default footage from analogue cameras is interlaced, which means for usable progressive footage or photographic evidence you loose half the resolution to begin with. In the end, best case scenario usable footage is 288 pixels in height, upsized/stretched to 704x576.
Just google analogue vs IP images on google. I know budget sometimes dictates old tech, but believe me, unless it's a completely unlit dark corner with most awkward visual access/angle, even 30 quid logitech 720p USB webcam duck taped to corner of the window and hooked up to old laptop/box running the simplest software, be it Zoneminder, Ispy or even YawCam will produce more usable security footage than the best analogue camera connected to the best DVR you can buy at the moment.