From 1971 to 1976, he was an aspirant in Pushkin, Saint Petersburg, located in the Northwest Region Institute for Agricultural Engineering, Electrification Design and Technology Research.
From 1957 to 1958, Kostabi was an engineer-controller at Orava MTJ, a manager of automotive industry from 1958 to 1960 at Orava State Farm and chief engineer officer, deputy director, director at Veriora State Farm from 1960 to 1990.
He has participated in various social professions in the form of trade unions and party organs, judges, folk song-writing, social road inspectors, people's voices, the XXVI Committee of the EELC, the Võru County Council, the EELC Petseri Peter Church and other work and activities.
He was the writer of the book "Kaks otsustavat päeva Toompeal" (1996), compiler of the collection "Omariiklust taastamas" (2001), compiler of the book "Baltimaad hävitatud omariiklust taastamas" (2009), and the updated version of the collection, "Vundamendi taastamine iseseisvale riigile.
Dokumente, materjale ja meenutusi Eesti Vabariigi Ülemnõukogu tegevusest aastatel 1990–1992" (2012).