Lighting For 50 Gallon Tank

[Axyadbad]

I've been fortunate enough to get my hands on a 50 gallon tank with filters etc for £50 off a mate

 . But its got no light unit with it. I'm just wondering what the cheapest option would be to get it sorted..I fancy a heavily planted tank. 
 
Also...the dimensions of the tank are 36"x18"x18" and I'm just curious what 'big' fish would be suitable as a centrepiece?
 
Cheers for any replies!