Would also recommend selling your existing skyline and getting a turbo one.
But contrary to Security's opinion that you will get 4k for yours, i think NA skylines are holding their value far better than turbo models, due to p platers being unable to get turbos. Have seen a few NA's for sale for 8-9k. So if you are lucky, you may even break close to even if you sell yours and buy yourself a gtst.
but either way, you will save yourself a fair whack of money if you just buy an already turbo'd skyline.