Just to avoid any confusion, a slight correction it is Merrivale Range, not Mereworth Range.Good on you, I hope you have a good camp on Dartmoor. I walked out to Wistmans Wood and the surrounding Tors last week and it's a lovely place to spend time. The advice already given about checking the camping map and firing times on the ranges is well worth following. For your first time, I'd recommend that you visit the Tors in and around the Mereworth Range as it is very accessible (Great Mis Tor has already been mentioned), lots of parking and easy to navigate. That area does get busy at the weekends though so do bare that in mind.
Asking about sleep systems will invariably result in a series of very different responses. It's a personal choice for you based on how you sleep and your physiology. In a nutshell, the thing that you really need to deal with on Dartmoor is the exposure to wind and with that the cold damp air that you often get up on the Tors. For that reason, I'd suggest going up a season in your sleeping bag (compared to what you might ordinarily use in lowland conditions). Sleeping pads are important, but don't overthink it. Some folks advise against foam mats, I disagree and have used nothing other than a basic foam sleeping mat for several years. I'm not a particularly warm sleeper and have never felt them to be cold. Obviously not as comfortable as the inflatable pads, but they're cheap, reliable and your back soon adjusts to the harder sleeping surface.
Hope you don’t mind Barney.