Only about 20% of all 355's produced needed the valve guide upgrade and any problems would have shown up in the first 2 years of ownership. So you don't need to worry about that anymore, it is highly unlikely you will find a 355 that is going to show problems in this regard (see the FL buying guide for more info).
Not sure what you mean by the header upgrade, I haven't heard of that before.
As far as I can tell, ALL 355's, ALL years, are at risk for the valve guide issue. The percentages listed are rubbish, as many of the nicer 355's have low mileage and are just now getting the problems. My 1995 had it's ceramic cats go bad and literally disintegrate at 12 years old and 7000 miles. The 13000 mile 15 year major service failed the leakdown and required new valves. I planned to replace the headers at the major to try to prevent future valve troubles, but I apparently was just unlucky. Competizione says that they never saw a low mileage 1995 need valves......now they don't say that any more! And of course, now that the car has been corrected, I have no interest in selling it! I think many are bailing from the big services because they don't want to put the money in if they aren't planning on keeping the car. If you plan on keeping the car long term, by all means put up the money and make it right! My car runs better than the day it was new, but it was costly!
ALL 355 headers are going to eventually go bad. The stainless is too thin and weakens with time.
OEM replacements are costly and will go bad too. They are expensive , but the TUBI 4 into 1 with heatshields are the best replacements. The other aftermarket ones have some controversy as to whether their heatshielding will not kill the alternator and other vital parts.
Just to depress everyone else, the 360's are starting to get header problems where the precats go bad and suck crap into the engine requiring megabuck repairs! I will plan on TUBI headers for our 360 at it's major coming up next year......SIGH!