Tolga, GT frame is nice, however my model is 2006 and I will probably not recommended, unless u can get it really cheap. The main reason is the headset is not angled enough to accompany fork bigger than 100mm, as it will affect handling. Also its 1.1/8 straight headset, so u can not fit tapered forks in it (unless you fit in external headset, which will again affect handling). I wanted to upgrade my old rockshox Judy and that gave me a proper headache. Wasn't willing to spend too much on mud bike, so it's partially my fault, but to get descent 1.1/8 fork you looking for around £200. I end up just cleaning Judy and spent money on reverb lol
As bugeye posted here, I will probably go for onone frame, which has more modern design and will be easier to get components for. Check also commencal frames, some nice discounts on 2012 models.
It's all about budget, building bike can be more expensive on first, but you will not have to replace components you dont like.
Love the kona handlebars....it looks super comfy