The "rule" is 100+ pixels per foot for a great face shot, depending on lighting. Try the
www.ipvm.com/calculator and plug some camera specs in to get an idea of what length lens you'll need to get a good face shot outdoors. There are too many variables in a 27 cam system to recommend a good cam for each one. Some rules of thumb- one cam per entrance located at or just a hair above face height. 3.6mm or 4mm is good for that. 2.8mm gives a wider view but is unreliable for identifiying anybody that doesn't very close to the door and can cause other problems, particularly at night. Covering a car in a driveway, even parked close to the house, should have at least a 6mm. Turrets or bullets work best outside if you want to use the built-in IR. A bunch of bullets outdoors, except for the mini bullets like a DS-2CD2042-I , tend to make your place look like either a prison or Fort Knox. You may or may not want this. Skip domes outdoors unless they're mounted right under an eave and you'll be using an external illuminator with the dome's IR turned off. Indoors, domes are good. Don't assume you'll always want a 2.8mm or 3.6mm lens indoors, particularly if you have large rooms, although the wide views are nice. Always think of what job you want the camera to do. Wide views tell you WHAT happened. Longer lensed cameras trained on chokepoints and areas of interest (doors, driveways, gates, your Monet, Porsche, etc) give you the money shot of WHO the person was, particularly if they're strangers or visitors. You'll be combining the multiple views for evidence so as long as you've got a decent shot of what they're doing you can combine it with, say, the high quality face shot of them coming in the front entrance or through a natural chokepoint like a hallway in your house.
As far as what to get, look at either a Dahua or Hikvision (commonly branded LTS in the States) system from some place like Wrightwood Surveillance or maybe Nelly's. Just make a map of the place, use my rules of thumb and the IPVM calculator, and pick out the type of cams with the lenses you need. Nelly's doesn't always offer much with longer lenses, so Wrightwood might be a better choice if you want to mix in 6+mm cams for outdoors. Either that or just jump off the deep end and order cams and NVR direct from China. You'll get better camera and lens selection and better prices but mediocre support and a crappy warranty.
Personally, I'd prefer running the cams into PoE switches, possibly locating a few switches around the place in strategic locations (say one in a garage and running close cams to it, one in the attic on the other end of the house, etc to simplify wiring) and running them into a gigabit switch into your NVR or a computer running software of some kind. I prefer computer solutions but for a newbie doing this with an NVR isn't a bad thing. If you don't like it, you can always replace it with a computer later.
Blue Iris is the software of choice here, but it's not dead simple and a 27 cam system will probably crump even a screaming computer if you want to have 4mp cams running 30fps. For network requirements, figure maybe 8 megabits/s per 4mp camera. Dividing the cams into at least 4 10/100 switches would be good, or just go GigE PoE on everything and be done with it.
If I wanted to install a 27 cam system with all the bells and whistles, I'd probably call Avigilon and see if they'd sell you the licenses if you did the install yourself. They've got excellent software that I found easy to understand and use and great support. You'd be dependent on the in-camera motion detection which means you'd have to use cams supported by Avigilon. I know they support Hikvision cams. I'm not sure if they've added Dahua support since the last time I used it. Hikvision has the advantage of greater industry support but I'm liking Dahua cameras more and more lately for a number of reasons. Avigilon also sell their own excellent cameras but they start around the $1000 range per camera. Put in outdoor cams with external alarm inputs and use PIRs if you want decently reliable motion detection regardless of brand.
As for rack mounting switches and NVRs, you can certainly do that. Match your NVR with the camera maker and get the model that'll handle all of those cams and has LOTS of HD storage. 27 cams will eat terabytes of storage like a fat kid eats candies. If you're doing all of your network stuff yourself, I'd recommend against putting in bedroom cameras unless you don't mind seeing you and your kids on porn sites. Mistake-free network security isn't for newbies, particularly if you must have remote access and run them off the main system. Running everything off a VPN will certainly help cut down on the possibility of unauthorized access but you should have professional help setting the security side up. I'd isolate the system from the Internet while you're installing things and getting it the way you want it, and then have the pro come in and set up your security for you. If he says that port forwarding and unique passwords are good enough then get another "pro". You'll get the satisfaction and cost saving of doing most of the grunt work plus the peace of mind knowing that your wife has someone to yell at besides you if your system ends up not being secure.
