A tarp, sleeping pad, and quilt is pretty simple and will keep you warmer than sleeping in a rain suit. Sleeping in the rian in a rainsuit would be a truly miserable experience, even if you don't end up with hypothermia. You will end up completely soaked in short order. Rain gear is designed to shed water downward when you are mostly vertical. When you are horizontal, the water will have no trouble getting in.
A better idea would be to get an ultralight poncho/tarp combination. You can wear the poncho for rain protection during the day, and pitch it as a tarp in the evening. One piece of gear with two uses. It would be a bit cramped and not have room for much gear though.
Don't give up on the idea of a sleeping pad and bag (or quilt). Sleeping on bare ground in any but summer conditions will suck the heat out of you. Putting together a bed of dry leaves and pine needles is great so long as there are dry leaves available and they are not full of insects. Otherwise, a thermarest is pretty handy and gets a lot of use.
BTW, what you do in a survival situation is much different than when you plan ahead. It's foolish to eschew proper equipment unless you know exactly what you are doing. People who specialize in playing survival games generally have considerable experience and practice under controlled conditions. Even then, you'll probably find they spend quite a bit of effort making shelter. Putting up a tent would be much simpler.
If you want to experiment, then give it a try, but keep your regular gear handy in case things don't work out.