With only one completed comparable, I wouldn’t claim a defensible percentage range yet. Start with that sale, confirm that its area is measured on the same basis, then compare price per square metre without applying it mechanically. For condition, a separate allowance for the dated work is...
I would also separate the monthly building charge from property tax rather than deciding which matters more. Buyers may react differently to a recurring charge, a tax bill and expected renovation costs. Two studios at the same asking price may therefore have very different effective carrying costs.