It sounds like you do not need a high flying Digital Video Recorder, just something better than you've got. I do this for a living, and for a small (no offense) operation I would use this:
EZWatch Pro It is expandable, and a fairly low-cost option. (I am not affiliated with them, just put it in before for customers who had it, and now recommend it to potential customers who do not want/need the High Speed/Low Drag stuff we do sell)See their web site for the pricing, features, and system requirements. I have put this in, and it is a nice system and a very good value, bang for your buck wise. I think the business grade version will record up to 7.5 frames/second per camera or lower, depending on your requirements. The professional grade will do higher frames/sec. but is more expensive. This will also record audio if you have it, but it is one less channel your could use for video on that board (4 channels total per board, either video OR audio, 1 audio channel/board) Software is also accessible over a network, which is really cool. It can also email you an alarm if detected, i.e. motion. Check their website, lots of features.
As far as replacing your cameras, only do it if you want to or don't like the quality of the camera, it is not required, don't let anyone tell you it is, video is video. The software mentioned above records at 320x240 resolution, pro is 640x480 resolution. So it depends on the resolution you want and the quality cameras you already have. They sell adapters for the cabeling of the cameras, if they don't match the boards (BNC I think, been awhile), at any decent electrical supplier.
You could put the new cameras you want in yourself (Radio shack, Sam's Club, and EZWatch also have these, don't know of the quality, or of the resolution/quailty you desire). It really isn't that hard to put them in, if you can mount a shelf, run a drill, focus a camera, and hook up a stereo you could probably do it - it's all the same, camera's mount very easily, and the hook up is a cinch(don't know your exact building and where the cameras are going) Also EZWatch's customer service for their software is outstanding.
For a pro to do it, price varies for cameras utilized ($100 on up to many, many thousands), distance of cable run to recorder from camera, and how many walls it must go through, and how hard it was to mount said camera. I am in no position to give you even a rough estimate, without these details.
Hope this helps. I have made some glaring assumptions and made some conjecture. If I am way off, disregard this.