When you say gas engine, do you actually mean gasoline as in the large engines for big planes? Or do you mean glow engines which are used on the smaller planes that most of us fly most of the time?

Generally, you choose the plane first and then pick the engine based on what it needs. I'm not aware of any combos that include fuel tanks, because you are supposed to choose the tank that fits your model.

Gassers generally come with at ignition. Most people don't use electric starters with gassers because electronic ignitions make them so easy to hand start.

So my advice would be to choose your plane first, then get the engine and accessories that it needs.