The KPIs you choose to include in your proposal comes down to what KPIs are the most important to your client. If they want to improve site speed, then you may set metrics around loading times for specific pages. Or, if they want to rank for a specific keyword, then you could set a KPI for the ranking over the keyphrase over time.
The point here is to work with your client and create a highly personalized proposal with your template as a base.