I agree with everything Scott said - a hayfield doesn't need to be and normally isn't totally flat, and it may have rocks or even some small branches lying around in it. Numerous tractor passes also tends to leave ruts. If you really want to do this, the fastest way (apart from hiring it out) would probably be to spray the field with roundup and then use a power rake on it, followed by a lawn-type seed drill or a broadcast seeder followed by a chain harrow. As noted compaction may be an issue, which would be best addressed with a subsoiler - something the farmers probably have, and probably not worth buying your own if you don't have other fields you're going to be farming.
Now for the intrusive question - why on earth do you want to turn a working hayfield into a 10 acre lawn? That's a huge area to mow for no apparent benefit, unless you're going to have 4+ soccer or football fields on it. I'm sure you have your reasons, but have you considered just having the farmer keep using it, maybe renting it from you and planting whatever crops he wants (including continuing haying if he wants)?
Around here people will do hay on fields as small as 1 acre, and get up to maybe 3 tons of hay on just the first cutting from a 1 acre field. It may not be useful for you, but the farmer could probably sell it.
As noted, most hay grasses don't make good lawn grasses. Even if it were fescue or ryegrass, it would probably be a hay type that grows much faster and taller than turf types. So you'd be mowing twice a week instead of once a week. If it's alfalfa or bromegrass it's not really an option for something you would want to call a lawn.
If you do want to mow it, I would third the advice to use a brush cutter but sharpen the blades so it works basically like a finish mower.