You should most definitely diagnose before you buy parts. There is NO WAY to know what you need until you take it apart and inspect.
I would be very surprised if you need lifters or valve springs if the car has been reasonably well maintained and not driven past the redline frequently. Also, I would be surprised if you had to replace all the valves.
Does your doctor tell you what medicine you need before he examines you? If he does you need to find another doctor.