We now make hay on not much more than an acre. We have machinery from when we made much more, so still use that. I can't imagine contractors would do it for us even if we didn't have our own machinery, but they would do 5 acres. We absolutely could not make 5 acres of hay by hand, as it would take too long. Here in Scotland we usually get just one chance a year, and sometimes not that, to have a 5 day warm, breezy and dry window. In that time we turn our crop every day. To do that by hand in that time frame would need a large team of workers used to making hay by hand - it's exhausting work, especially in the heat. So you would need some type of tedder.
Whether it's worthwhile buying your own machinery depends. All our stuff is very second hand, some rescued from the scrappy, some rejected by everyone else at Lanark machinery sales, but Mr F is good at mechanic-ey stuff so he maintains everything. Last year our tedder broke finally and irreparably, so we actually bought a new one
It wasn't especially expensive, but if we had bought everything new, well, we couldn't have done that. Then we would be back at the end of the queue waiting our turn with the contractor. In that case, it would be cheaper to buy in the hay. Be careful though - one load of hay we bought in a wet year when we didn't get our own crop, contained so many thistle seeds that we have been taken over by the things, and attempting to eradicate them is an annual job.
We prefer our own grass as we don't use fertilisers so don't get staggers or bloat or whatever, and it's a very varied and nutritious sward which our sheep love.