1. You should verify the title deed about the legal ownership as mentioned in it, i.e., the name of the buyer and the seller in the registered sale deed/title document. Go through the contents properly to confirm the name of the buyer. If the company is the buyer then there is no need to worry about its registration.
2. Yes, provided the POA deed is a registered document. This is just an authorization letter to present the execution of registered sale deed on behalf of the power holder.
3. The power holder ha to sign the registered sale deed document though it is being presented by his representative.
4. The POA deed has to be a registered document.
As far as buying the immovable property is concerned it is always advisable to obtain a legal opinion from a lawyer before venturing into the proposed purchase.