My first question to you would be, what is the core value of in-store shopping for this group? So a pre- and post-shopping survey of elderly and disabled people's expectations, wants, and attitudes toward the in-store shopping experience would be the first step. Followed by a study, if it can be arranged, of some of the design changes they specify in the pre-study to see how they respond.
To further my research i tried to follow empirical method which involved observation as first step and interview as the next step. I tried to observe few elderly customers shopping and noticed few problems they had and to get to know their wants and expectations I had a short interactive session with them, which gave me an opportunity to understand their needs. As a result I could say that there can be few in-store attributes that needs universal design principles applied to give them better experiences
I think you should examine the theory of flow when you consider this question. Flow is defined as a pleasurable cognitive “holistic sensation that people feel when they act with total involvement." That sense of immersion allows individuals to be more engaged and to give them that better experience you are considering. You can find several good articles on Flow and other information online. Here is the citation.
Csikszentmihalyi, M. (1977). Beyond Boredom and Anxiety. San Francisco, CA: Jossey-Bass.
In line to the work that has carried out by you Nikhila,so far, try to do more literature review and look out for studies which match the in-store attributes identified by you. In the process you may also come across other in-store attributes from various studies that had influenced the targeted customers you are researching on. The researchers generally mention about the framework/ model used for their study. You could try to design a suitable questionnaire (either using the already tested model or your own framework) and carry out a pilot study before going for the full survey.
I am attaching herewith a pdf file of a study related to your area of study that I had come across for another purpose.Hope it may be useful.