Please help with the wording

I think the way to do it is:

> Equipped creature has ": Choose a land type. This creature gains landwalk of the chosen type [until end of turn?]."

You don't need to target a land an opponent controls because, well, that's the only reason you'd be doing it...

If you're happy to neglect the very few ways to attach Equipment to opponents' creatures, you could simplify this further to:

> : Choose a land type. Equipped creature gains landwalk of the chosen type [until end of turn?].

Also note that an equip cost of means it's quite easy to sling this between creatures. For you can grant two creatures landwalk before declaring attackers.