Huge subject. I'll try and cover everything with a few salient points.
only thing is not sure if I should spend my money on a basic heart rate monitor and basic wireless computer with cadence or just a heart rate monitor, or just cadence, or can these be combined, or do I really need these at all? Not sure where to start :o(
They can all be combined and ideally should be (because otherwise your bars begin to look like a cockpit) but the price can begin to escalate. Somebody else mentioned the Polar CS200 - well there's a cadence option too; the CS200cad. That would be my choice.
Question is, what are you going to do with all that information you're now getting?
Cadence: road cadences tend to be in the range of 80-100rpm. This is typically a lot higher than mountain biking, so will take a bit of practice to get used to. With a higher cadence and lower force on the pedals, you'll be able to go longer and further. As a side benefit, you'll end up with a greater pedalling efficiency, which will have a cross-over impact to your mountain biking.
Set yourself goals: those marathons and sportives you've got your eye on - book your place now. Without goals, your 'training' will have little purpose and you'll drift along doing a bit here, a bit there and opting for the sofa when you know you should be heading out.
Weight loss: huge area, but in short, this needs to be a combination of exercise and diet, with an emphasis on the latter. Keep a diary of everything you eat. Even if it's for just a few days you'll get a very good idea of what you're eating and where you can cut calories.
HR-based training: again, huge area, but I'll say this: all cycling plans are based on the concept of having a sound aerobic base, and you almost certainly don't have one. Your training, initially at least, should be based on doing all your work in your aerobic zone (known as base training). Setting your HR zones is another big subject, so to shortcut that, I'll suggest you just use the Maffetone/Allen method of 180 minus your age (http://www.markallenonline.com/maoArticles.aspx?AID=2) and stay below that level. Once you're more experienced, you can start adjusting your aerobic threshold levels and working to more specific numbers, if you want.
Your primary aim in the initial phase is to improve your input:output ratio. Input is HR (which should be a constant) with output being speed and distance. As your training progresses, what you should find is that your ability to go further and faster improves at a given HR. http://www.joefrielsblog.com/2011/01/comparing-output-and-input.html
Oh, I should add that working primarily at this level improves your body's ability to use fat as a fuel supply. Win-win 
I will disagree with all the people that have told you to 'just do intervals'. Intervals have their place in cycle training, but you need to be careful. Interval training is for honing your fitness, not building it. If you go from nothing to spanking yourself several times a week, you'll very quickly burn out.
So anyway, that's my advice FWIW. Keep your HR low, your cadence high and build your aerobic fitness.
