100 Rupees Stamp Paper enough for Registering 10 Lac Rupees Plot Land?

Sir, I want purchase a piece of land (Plot) in Chennai. The seller is Ashok Nandavanam Properties Pvt Ltd, selling 285 Plots. They are famous Real Estate Developers in Chennai. They are collecting 6.5 Lacs (Guideline Value 3.5 Lacs) for the said property. But Registration Stamp Paper, they are executing in 100 Rupees Stamp Paper instead of 11% Stamp Paper. They are telling, to all customers they registered with 100 Rupees stamp paper (that too only First Page only with 100 Rs. Stamp paper). They are telling even more valued (15 Lacs) property also executed with 100 Rupees Stamp Paper only. They registered the propery with SRO also. Can I purchase the Property from them? Can this document and Registration treated Valid Sir? Whether I will face any problem in selling the property at future date sir? Request your help?