Should I valuate the company by myself or should I turn to an attorney? What attorney should I look for? Can you provide links to exemplary, reasonable Term Sheets, you've already signed?
The first question cannot be answered as asked. The starting point is agreement on how the investors are to get their money back.
Neither you nor your attorney should attempt to value the business. Who you turn to depends on the business itself.
There are norms available even for pre-revenue firms but usually not term sheets for specific transactions.