There is a certain amount of subjectivity in the bracha of shecheyanu in that the bracha is said if the object you buy brings you joy.
Authorities dispute whether the berachah of shehecheyahu is entirely
subjective, depending on the subjective joy a person feels, or whether
the berachah includes an objective element, whereby if a garment or
item is not important, and does not usually induce joy, one cannot
recite the blessing. [...] Even if the item has some degree of
importance, such as a fancy shirt, it is sufficient for making the
blessing, provided that the person experiences subjective joy in
R Chaim Cohen here writes
The Shulchan Aruch (OC 223:6) and Mishna Berura (223:13) explain that
this only applies to items that are important and one is particularly
happy about acquiring.
Following the discussion in the comments, I found additional opinions supporting saying shecheyanu when experiencing mixed emotions, focusing the three weeks between 17 Tamuz and 9 Av. The following is extracted from the sefer Aveilut Hachurban by R Yoel Schwartz (from here, p. 75)
- According to Shulchan Aruch and the Arizal, one should not say shecheyanu during the Three Weeks. However, one should not "spoil" the joy of a mitzva during this period of time by not saying the bracha -
therefore at a brit mila or a Pidyon HaBen (even if they were postponed) the bracha is said. They also allow it for a (new) fruit that
will not be available after Tish'a b'Av
- The Vilna Gaon holds that one need not refrain from shecheyanu during the Three Weeks
So if buying the medical equipment (like a heart defibrillator I assume) brings someone joy, he could say the bracha (there would be a secondary question of shecheyanu vs. ha tov v' hameitiv if the equipment can help others). If the worry of never having to make use of the equipment removes the joy completely, I do not think it is justified.
As always CYLOR before attempting anything you read about here in real life.