Installing a new HR system - like SAP HR - takes a lot of work; and it requires consultants with a lot of experience to install it well. That experience will help you design work processes that work well both with the system and for your business. Those processes and the technical configuration supporting them are the foundation for your system and all that you will do with it in the future - and that is why it is critical to get it 'as right as possible' early-on.
It's similar to building a house - you want the foundation poured well and in the right shape to support the house you will build on top of it. You want the right number of rooms for the purposes you need, with the walls and plumbing and electrical in all the right places. Once all that is built, it's difficult and expensive to change it - take a wall out here and build a new one there, and it's going to cost you. It's the same with SAP HR - once you get the initial system and processes built, it's going to cost you to change it.
And if you built a poor foundation - the wrong size or shape, with cracks in it - then fixing that after the house is built is major work. Or if you decide that the layout of the rooms just isn't working, it can be a major project to rebuild them the way you need. The same goes for your SAP HR system and the processes you've built up around it.
I think the only way to get close to building your SAP HR system right from the start is to work with consultants who actually know how SAP HR is used by people to do HR, Benefits and Payroll. You need consultants who have both implementation and support experience, and preferably some real-live experience in HR, Benefits and Payroll. The support- and business-experience feeds back improved configuration practices and business process models into the implementation process. Consultants without that practical support experience have never seen how people live in the houses they've built!