The KPIs that you should take into account when building a sales team really vary. Every company (and I know my own portfolio companies do it) should do the cost-benefit analysis of how many months it will take to reach a breakeven point on the respective salesperson. And in every industry, it is going to differ. 

It's really important to understand how and what you're spending your money on. In terms of KPIs, in the hiring process, it is really important to fully disclose your expectations. And often startups can jeopardize the entire department by hiring somebody of junior level and not necessarily with a sales background. I think full transparency of expectations and having the person understand what is expected from them is really important. 

My favorite is the classic 30-60-90 strategy because I do think there's a learning curve for everyone. I think anybody who says otherwise is kind of kicking themselves. You have to give people the time to learn to get up to speed without burning them out.