A simple spreadsheet could connect both approaches. For each property, record the advertised price, completed price where available, transaction costs, financing assumptions, renovation allowance and ongoing management costs. Keep unknowns visibly blank rather than filling them with optimistic...
For completed-price gaps, use both the amount and percentage difference. The percentage helps comparison, while the cash amount matters for financing and renovation capacity.
Prioritise work that prevents a small problem becoming a larger one. Stopping active water entry matters more than replacing worn finishes, even if the finishes are what you notice every day.