I am inclined to agree with the diagnosis. However, the cost to do all new guides and valves if you take the head off to do the seals is not much more. You dont always have to take the head off for seals, though. The head is easy to take off. Why dont you consider a DIY? All you have to do with the head is take it to any automotive machine shop, where the guide and seal replacement is routine. Best of luck with the worthy endeavor of keeping this car alive.