The HRM Tri strap gives you all the running data as well working for swimming. Alternative;y you can wear both HRM straps in the water as the HRM Run is waterproof it just won't record the readings. You can also wear the swim strap for running you just won't get the additional running data such as foot contact time and vertical oscillation. To be honest the HRM straps chafe when I run and leave nasty burns so I ditched that idea long ago, I only wear mine for Spinning and cycling.
I would say that to achieve what you want you will probably need the following: -
Garmin 920 XT (I know there is also a Fenix but I don't know the model)
If you cadence sensor is ANT+ then that should work with the watch (I have one, it's great!)
For swimming you can either buy the HRM Tri or the HRM Tri it depends what sort of swimming you do, the swim is supposed to last longer in the pool whereas the tri will be affected by chlorine BUT you will get all the run statistics from it. You won't be able to see your HR in realtime unless you take it out of the water.
I haven't bothered with the quick release strap as I was worried about the watch part falling off in the quarry where I swim!
I had been spinning for a few months before I started running properly although I did do a Race 4 Life when I was at my biggest, that was the hardest 5km I have ever done! I ached for days afterwards, probably worse than when I ran a marathon. I think I must have lost a couple of stone before I started running.
I would suggest mixing up the running with some other form of lower impact training, it's really good to mix things up a bit too. I am now into (short!) triathlons so I still mix up my training now.
Remember to celebrate every achievement no matter how small it may seem too. Best of luck to you, you have our support.