Alright, so with what you wrote along side easy to learn then, RPGM is the choice. Do take note that RPGM is very limited in what can be done in terms of combat mechanics.
I want to give the players the option of going all out combat to attain these lands with a steady gradient of increased difficulty ie 3/4 battles then a boss fight to take the land.
Below are some reference games in terms of combat mechanics that RPGM can do,
- Zelda (top down hack and slash) do a google search on (The Legend of Zelda: A link to the past) (this might require you to dig around community plug-ins to achieve this type of combat)
- Final Fantasy (Turn base) (RPGM Defaults to this style when you make games with their engine)
- Metal Slug (Side-scroll shootem-up) You move from left to right or right to left and just have a bunch of enemies that you hack-n-slash. (This mechanic is fairly new in RPGM, I've only seen 2 games, can't remember the names, again you'll have to dig around a bit if this is a mechanic you want to implement, also requires a bit of coding to correctly position your MC in a side scroll position)
Or do you use your powers have you built them up enough to take the land by stealth.And to mix it up a bit can you use your powers to take it by stealth or do you need to enter combat to remove blocks eg powerfull mages witches or wizards that can detect and nullify your powers.
As far as stealth combat goes, there isn't much to say here, you're either spotted or hidden, and if you get caught you enter combat (which than transition into whatever combat you implemented) or run away. If player is hidden, than as many other stealth games, you instant kill the target.
I stopped working with RPGM a long time ago, I mean loooong time ago, so between that and now many plug-ins and changes might have come as well. Dig around a bit and see if the engine offers the tool to accomplish your needs. Hopefully this gives you an idea of what RPGM can offer in terms of combat mechanics. If this doesn't satisfy the combat aspect, then Unity/UE4 is the other option. If you have any more questions feel free to ask, I'll answer as best as I can within the range of my knowledge and experience.