1. There is nothing wrong in entering into a lease for 70 years.
2. However, since you will be committing your valuable land for such a long period, you have to necessarily consult an experienced Chartered Accountant to know about the value of the property, how the lease terms (like initial amount, monthly lease rental, increase in lease rent annually/biannualy or in every three years etc. and other intricacies if any.
3. Similarly, you have to get the help of an experienced property lawyer for drafting the lease agreement.
Yes, point 2 and 3 above will involve spending some amount on the Fee of the Chartered Accountant and the Lawyer. But remember, you are dealing with a very valuable property of yours. Therefore, incurring expenditure on CA and Lawyer will be worth and will not be a waste.
Pl do not think that without their help you will be able to complete the lease transaction,
All the bests.