See the seller can apply for PAN card online on NSDL and he shall receive details of same in a week. Further a sale deed can made and executed by your father himself as POA of seller and buyer himself and it can be registered and stamp duty and registration can be paid.
And further getting Aadhar is not difficult the person can be taken to nearby aadhar centre for 5 minutes for biometric and eye scanning and he will get the Aadhar.
And at least get a agreement to sale notarised by taking notary at his place so you are safe and can claim property from his heirs if anything happens to him.