The concept of sustainability, and especially sustainable development, is among the most ambitious and controversial concepts in the scientific literature. Knowledge and research concerning the condition, development or transformation of sustainability has become not only the original means of the generation of socio-economic science knowledge. It is also an alternative for the analysis of especially sophisticated development problems – e.g. the problems of survival, effective change and avoidance of huge losses pertaining to such complexes as cities, countries or regions. Finding the ways of such knowledge conversion into the field of science is complex, but there is no alternative. Research of sustainable development has already become especially prevalent, and, thus, the objects of cognition should be structured, possibilities should be consolidated, and the efficiency of the use of resources must be elevated. The first challenge of sustainability research conversion into the sustainability science is a thorough mastering of the systemic research technologies, as well as the development of the principal methodology of systemic research while evolving the possibilities of civilisation survival on Earth. The objective of the research is to find an adequate quantitative measure of complex system development sustainability and investigate in detail the type of development of a small country or region that could be named as a realisation of sustainable development possibilities. The methodology of the research includes the application of an adequate portfolio model, stochastic optimisation, and systemic analysis. The conclusions obtained by the research state that development possibilities of a small country should not be evaluated only in terms of economic-ecological aspects; more components, such as politics, integration, marketing, social-demographics, creativity, religion, innovation, finance, and investment should be included, which could be logically divided into four subsystems of country universal sustainability. The research also presents the adequate composition of efficiency and reliability of the general effect pertaining to the activity of all the subsystems, as well as the optimal allocation of marginal investment unit among the four subsystems. DOI: http://dx.doi.org/10.5755/j01.ee.25.5.3797