Well First and foremost the wording is terrible. It should be something like: PLayers can play nonland cards any time they could play an instant. Or something...
Anyway the second line won't make any sence after the first line has taken effect. If you kan play your spells as instants, then you can also play them during the combat fase.
Third: It is overpowered... An enchantment that only costs

to play and has this great effect on the game is way too broken... It should cost at least 1

or maybe more realistically 2

.
Oh and by the way. Try to compare your card to Vedalken Orrery.
- meanee