Buy your friends turbo. You will be glad you did. Then if it need rebuilding, you can get the HP you are looking for. I don't understand why you would want to buy a used engine. You are still going to have to go through it and replace all the seals and while you are at it, you maight as well check the bearings, then you maight as well check the cylinders and pistons for wear, then you might as well have the head checked and probably rebuilt.........so, to me you are better off rebuilding what you have. Unless there is a hole in the block.
My daughter has my old 87 N/A Supra. I rebuilt it and each time I drive it, I wish my turbo had that kind of lower end torque. It just really runs out on the upper end. I think more compression and better breathing would turn the N/A into a nice drifting car. Lighten it up and stiffen the suspension up and have fun...... or sell it. That would give you some money to put more power to your new TURBO car!
Russ