Before you contemplate cutting the grass a few things to consider- if you cut and bale for hay the value and quality will be lower due to the thistles. How wide are your gateways- my OH has a cracking small baler which is always in demand but it is a very wide piece of equipment and needs a wider than normal gateway to get in to the field. Have you got storage for said hay if you don't manage to sell off the field? It might be worth asking the farmer to come have a look- he will be able to advise whether it's worth the time, effort and expense of making hay or whether it's just worth spraying the field for weeds/thistles and topping field after. Hay is such a labour intensive crop- cut on one day, turn for the next 3 or 4 days, bale and then bring in on trailers, it may not be worth the time if your end product isn't great quality.