@John Woodington
Caveat, i am not a land investor (yet), but grew up around farming, and one day want to go back to it. i can provide a little insight on your questions just based on my own experience.
1. Farmland is not the best investment, numbers wise. whether you are working the land yourself, or renting to others generally it must be debt free to be profitable. Trust me, as someone whose life long dream is to get into farming, its very difficult to make the numbers work if you have to mortgage the land. even for land owned free and clear, expect returns in the single digits. As for rent amounts, there is no single combined source to determine land rents that i have found. Im currently looking for farmland for rent in georgia for my expanding cattle heard, and its difficult to determine what to expect. my best advice is talk to local farmers in the area. most farmers of any size rent some amount of land, and should know what the going rate it. If you dont know any, your local Natural Resources Conservation Service (NRCS) office can most likely help you out.
2. Yes, being landlocked decreases the value. but are you landlocked from a legal sense (your property does not touch an access road) or do you just not have a proper driveway? If the former, you will need to work out an easement with a neighbor to get access to the land. the best way to do this is to buy the easement, so there are no legal issues down the road. this would give you the most value for your land. If no neighbor is willing to sell, you will need to work out a right-of-way.
as far as rebuilding the road once you have access, your local NRCS rep can help with that. They can help you determine soil suitability, lay out the road and grading plan. you will then need to hire a grading contractor to build the road. You will also want to check for any needed permits in your county.
3. I believe you can work something out with your cousins/siblings now, but i would highly encourage you to get an attorney involve and get it property documented and recorded. The easiest path - everyone agrees to sell their share to you, yall all agree on a price, and get your parents to write this into their will. or have them set up a trust and name you as the beneficiary. Either way, get a lawyer involved. These things can get nasty, and no one things probating a will can cause a family dispute until it happens to them. No matter what the path, you will have to agree on a price and buy out your relatives, unless they are generous enough to give it to you.
On that note, valuing land is not very straight forward. unlike houses, which generally have the same components, land can vary greatly. Irrigated vs non-irrigated, are wetlands present, are their conservation easements, soil type, access can all play into the value of land. in general, its worth what someone is willing to pay for it. What someone is willing to pay for it is based on its future use. does it have future potential for commercial development? if so the land will be valued much higher.
There are a lot of what ifs and family dynamics at play, but if a buyout is in your future, you may want to wait to make any improvements until after. hope this helps.