The liablity to pay service tax is on the service provideer (builder in this case). Service tax department cannot raise demand on the buyer of the property to pay service tax on behalf of the builder.
Both the tax free bonds notified by Central Government currently i.e. NHAI and RECL have the same lock in period of 3 years and rate of interest of 5.25%. So the capital gains of Rs. 10.50 lakhs can be invested in any of the bonds to claim exemption under section 54EC of the Income Tax Act.