Hi Gorapco,
I have a small hay operation like yourself, and run an 8 foot discbine 408 NH with a 70hp tractor without a problem. I would definitely use a discbine over a haybine, as you can run a discbine a lot faster (ground speed that is). If a discbine isn't within your budget, then a haybine over a standard disc or drum mower for sure. The conditioner is a godsend for drydown time, as an example, my neighbour runs a standard disc mower, no conditioner, and he needs an extra day or two dry down over me. I could easily swing another foot or two behind the 70 horse tractor, but the 8 footer is great for gateways etc. Having said that, my country is all flat, with no rocks. If you have large rocks, I would probably choose a haybine, and if you have hills, I wouldn't go any bigger.
I hope this helps.
Aaron.