Most of the used sickle bar mowers I see aren’t worth $750, but there are a few nice ones out there. But personally, if I could get a usable haybine for $1000 or so (and around here I can), that’s the direction I would go. It doesn’t seem like it now, but that little extra money will be made up in time that you’re NOT sitting on the tractor mowing at 2mph.
With only 5 acres, you’d be money ahead to just buy hay and pasture everything. I’m not sure you’ll ever get your money back with only 5 acres of hay, unless you plan to lease hay ground.