4x Elvish Mystic is a must. Consider getting on the full ramp plan with Garruk, Caller of Beasts , Nykthos, Shrine to Nyx and as many mana dorks/fatties (Kalonian Hydra , Mistcutter Hydra , even Worldspine Wurm as you can cram into a deck.
If budget is a concern you can still play the Mystics and cut the Limited filler cards like Akroan Horse and Opaline Unicorn for better cards. Ordeal of Nylea also seems weak here, as you are unlikely to have solid, survivable early attackers (or +1/1 counter abuse) to pop the Ordeal.
Honestly, Mono Black is a much better shot at getting out Colossus. People often underestimate the ramping power of Crypt Ghast, as he allows for a turn 5 Colossus. Take into account the spot removal that Theros has given us, and I'd say you have a good start for a black Colossus deck. Green does it well too, but without farseek and arbor elf, it seems a lot less powerful.
Hey man, I do love the concept of getting him online and swinging in for easy lethal! There is however a small problem, while Indestructible is great and nearly unkillable, you have to look at some of the cards that WILL be seeing play that can take him out, and if he dies, there goes your win-con for the most part. Selesnya Charm (I run 3 of main board), Detention Sphere , Banisher Priest and Chained to the Rocks are all going to see a lot of play in Standard, or I should say already do, with the addition of Chained. For that reason I wouldn't recommend playing a deck relying so heavily on a single card. That being said however I totally dig it and hope it can work, but if you're trying to get competitive this might not be the best route! Best of luck and +1 from me, cheers!