I have access to mains water in the greenhouse.
Thats fine but do you have access to electricity?
So far as I know misting systems are electrically operated.
How many times should they be misted a day (cost of wet leaf system makes it a no-no!).
??? This sounds like you are now talking about a 'drip/seeping' system which is a totally different thing.
Do I also need a heat mat or cables.
Again do you have electricity?
If you have then I would recommend either a mat or cables, I use a cable on my hot bed and my propagator is effectively a mat.
So if you have electricity a misting system is more common when taking cuttings as opposed to sowing/germinating seed. So a drip feed system would suffice particularly if you want to use capillary matting.
If I mist I generally just use a hand pump spray. The number of times I use it per day depends a lot upon the weather & temperature.
What you want to avoid is overdoing the spraying, this can make matters worse i.e. botrytis comes to mind.
Sorry I can't be more helpful than that.