Everyone's going to give you different opinions on fluids and mods.
But I will caution you from using Seafoam. I've used it on 3 high mileage cars in the past with less than optimal results. Sometimes the carbon deposits in certain areas of the engine is what keeps the engine "tight". You can experience loss in compression, gas mileage, defective sensors, etc from doing Seafoam. On the other hand, I've known people that had decent results. It depends on the state of the motor and whether you really "need" to do it or not. I would just try running some Chevron Techron through it for a couple gas tanks first. That should at least take off the carbon deposits from the valves if you have any.
As for synthetic, visually check for any existing leaks and loose bolts. If you find none, then feel free to try synthetic but be prepared to switch back if you start finding leaks or odd noises from bolts loosening up in the motor.
Do you personally know or trust the work that's been done to the motor? The reason why I ask is I bought a car with a newer head on it and it was done all wrong with the bolts not torqued correctly. Overtime, guess what happened? :rolleyes: