Hi Ms Sally Wong,
Yes, single Singapore PR is not qualified to purchase a resale HDB unless a married couple (both PRs or with 1 SC).
At the moment, the price of a typical studio or 1 Bedroom (less than 500 sq ft) varies. It all depends on tenure (Freehold vs leasehold), locations (e.g. prime District 9 vs outskirt District 19), amenities, floor height etc. In contrast, buying a new launch may have more advantages. It all depends.
Yes, the older the project, the price may be lower, but you have to consider the conditions as well. And not forgetting if the project is a 99 years leasehold, if it's too old, you may have difficulty on the bank loan.
Perhaps you may let me know your exact requirements and I can then advise accordingly. Thanks.