my sister went from nothing to the Great South Run following the C25K programme and then on from there.
It's got some great advice for beginners. But I would definitely invest in an HRM so you're not overdoing it to start with. Better to stay within yourself and run 3-4 times a week than go all out and knacker yourself so you can only run once a week.
Having a goal is a great idea, but you need to have a plan to get there as well and have it laid out in meaningful chunks.
basic HRM models - The main onese are Garmin, Polar and Suunto - I've tried the first two, have settled on Garmin for my own reasn
Good reviews here -
http://www.heart-rate-monitor.co.uk I've bought from these folks in the past although they no longer sell direct.
Garmin FR70 - HR monitor but not GPS, can pair with a footpod to give you distance
Garmin FR 110 adn 210 are the entry level GPS tracking watches
Polar FT1, FT2, FT4 - more basic, no GPS, no pace alerts, HR and time only only, but good enough - I started out with the FS1 or FS2 many years ago - perfectly adequate but my percieved needs changed. They now do GPS versions as well.
Suunto I don't know much about, never had one
Also look at
Garmin FR 50 - second hand now as discontinued, but you'll find on E-bay and the like, I had one of these, was very good
205 - same again, you may find them on e-bay from those who've upgraded
Avoid TomTom watches,they're ****