These questions cannot be answered properly without reading the easement. You should obtain a copy of the easement and have a lawyer review it.
Easements normally allow the user to perform necessary maintenance and repairs.
I don't understand your last question. You really should have an experienced real estate atorney look over everything for you before purchasing. it is much easier to get easement issues sorted out before you purchase. If you wait until after you purchase and you run into a problem, it will be much more expensive to address.
Without seeing the easement, it is hard to give a complete answer, but there are some basics that will give you a place to start.
An easement divides the ownership of the easement property into two estates, the dominant estate and the servient estate. The dominant estate is the right to use the easement, and the servient easement is the ownership of the land subject to the easement.
As a general rule, in a conflict between the dominant estate and the servient estate, the dominant estate wins. For example, the owner of the servient estate cannot put a fence across the easement unless he also puts a gate up for you to get through. If he locks the gate, he has to give you a key.
A typical easement in your situation would just be for ingress and egress and perhaps utilities. That would give you the right to improve the easement to perform its function as long as you did not exceed the limits of the easement or unreasonably interfere with the servient estate. You also do not have the right to use the servient estate to do the work.
As you describe your situation, it is highly likely that you would have the right to grade and gravel the easement. Paving it might introduce other questions. I would have the easement boundary marked by a surveyor before starting work to make sure you don't trespass.
The vast majority of access easements are appurtenant, which simply means that they are permanent. If you sold the property, you would also be selling the right to use the easement. If the neighbor sold the property, he would be selling it subject to the easement.
I would start by reading the easement itself to see if that answers your questions. Most easements are written in plain language, and you may not need an attorney to understand your rights.
I also suggest that before purchasing the property, you knock on the neighbor's door and explain what you have in mind. He may be willing to share the cost, and you will know for sure that you don't have a problem on your hands. Give your neighbor plenty of notice before doing anything on the easement out of courtesy if nothing else.
This is legal information and not legal advice. The final answer to your question will depend on more facts than you can include in your question and some that you probably would not think to include. Treat this as a starting point, not the answer.