Wow -- 20 stations at 18 hours/day is a LOT of recording :-) ... but not difficult.
However, whether this is reasonable depends somewhat on what you mean by "... good but not too expensive ..." ==> you'll need a good bit of hardware.
A single 1TB drive will hold about 650 hours of good quality recordings using MPEG2 to record. Most hardware encoders use MPEG2, so that's what I'd plan on. As I noted before, you CAN transcode the recordings ... but with the volume you have in mind and the amount of CPU power you'd need to do it, it's not worth trying to set up all the computers you'd need just for the transcoding => it's cheaper to just buy plenty of disk space. Since you want to record 18hrs/day for 30 days for each channel, I'd just plan on one 1TB drive per channel [18 hrs/day x 31 days = 558 hrs ... which is close to the capacity of the drive].
For 20 stations, I'd build 5 systems with 4 tuners each [You could also use 4 systems with 5 tuners each ... but with the amount of disk space you need you may have a problem fitting enough disks]. In each of the 5 systems, I'd put two dual-tuner cards with hardware encoders [Hauppauge PVR-500's would work nicely], and a system hard drive (this can be a modest size -- perhaps 250GB) and 4 1TB drives to store the recordings. I'd install Beyond TV [
http://www.snapstream.com/products/beyondtv/default.asp ] and set it to span its recordings across your four 1TB drives.
If the case you're using has room for more drives, you could install 3 or 4 dual tuner cards and record 6-8 channels per system ... as long as you have 6 or 8 1TB drives available. But you'd need to be sure you have adequate cooling for such a loaded system ... and a hefty power supply. Snapstream (maker of Beyond TV) built a ten tuner system you can read about on their site => you could even built two of those ... but if you did that, you'd need to build a separate storage server with at least 20TB of storage space. That IS an option ... you could use a SuperMicro 24 bay hotswap chassis and a good 24 drive RAID card (e.g. an Areca 1680) and build a 22TB RAID 6 array. With a GB network connection to your recording systems, this would have enough bandwidth to support what you want to do. The storage server would also give you protection against drive failures ... since a RAID 6 array is tolerant of up to two drive failures.
That should give you some food for thought to let you design your recording setup ... and some idea of what the cost will be. Basically you have two choices: (a) 2 recording machines and a large storage server; or (b) 4 or 5 recording machines with embedded storage. In either case you'll need at least 20 1TB drives and 10 dual tuner cards ... and you'll probably want spares of each.
