wow, tough call. The car is obviously a good one, having run and lasted that long without major issues (Im assuming). TO some extent, your question is very valid because you dont want to replace one thing, and then after spending the money, have something else fail. But if youre going in for TB, etc., you may be able to do an assessment and identify if it looks like things are good enough to justify trying the HG.
Id give it a shot if it looks like things will come apart and go back together OK. If not, Id probably not sink the cost or time into the TB either...
Have you done a compression and leakdown test? That may be most telling for you to make a decision off of.