The Shulhan Aruch (Orah Haim 223:4–5) writes that when one acquires certian items, one recites “Sheheheyanu”, and if others benefit from it as well, one recites “Hatov Vehmetiv”. Nevertheless, it goes on to say (ibid:6) that items which do not bring one significant joy, such as a night robe or ordinary shoes,do not warrant any blessing. What is and is not considered significant is obviously open to interpretation and is subject to debate among the Poskim. For example, Rabbi Alexander Mandelbaum (Vezot Haberacha, pg. 166) says that a hat, a wig, a Tallit, a suit or a dress are examples of significant items that would warrant a blessing upon their acquisition. Rabbi Moshe Feinstein (Igrot Moshe, vol. III, Orah Haim, § 80) writes that one may recite “Sheheheyanu” after buying a car, and if others will benefit from it, such as family members who will be transported in it, one would recite “Hatov Vehametiv”. On the other hand, Rabbi Moshe Malka (Mikve Hamayim, vol. IV, pg. 31) reasons that when one purchases a car, one does not attain the level of joy as when one actually starts to drive the car, and that when one finally drives the car, it is not considered new. As such, he writes that one should not recite “Sheheheyanu” on a car. This reasoning is questionable, as people tend to be able to drive the car soon after its purchase, and thus, one may rely on the opinion permitting the recital of a blessing. It should be noted that the blessing would also apply to purchasing a used car, but not to a leased car.
Not withstanding, the Ben Ish Hai (Parashat Re’eh, § 5), citing the Magen Avraham (O.H. 223:5) writes that besides, a new suit and the like, the common custom is to not recite a new blessing on new items. Some explain the Ben Ish Hai’s position by saying that it is unclear what is considered a significant item and that it is a subjective interpretation.
Interestingly, the Magen Avraham (O.H. 175:???) as well as Rabbi Ben Zion Abba Shaul (Or LeZion, vol. II, ch. 14) write that whenever one has a doubt regarding the blessing of Hatov Vehametiv, one may have it in mind when reciting “Hatov Vehametiv” during Birkat Hamazon.
Summary: There is a Halachic basis for reciting “Sheheheyanu” or “Hatov Vehametiv” when one acquires new items such as a car, a piano, a suit etc.