I am agreeing that cattle and sheep are okay to mix, but you might want to rotate your pasture so that you get the benefit of one speicies cleaning up after the other. Simpler to run together except if you have cattle licks out in the field these are toxic to sheep. Cattle can live out all winter, our Herefords do with silage brought to them. On a small scale I would advise housing in the winter to save your ground from damage.
Have you thought about just rearing a few ordinary market calves to start with as a trial? When you choose your breed I would recommend Herefords or Aberdeen Angus. They are docile, hardy and good mothers. Steer clear (no pun intended) of limousin x cattle. I am sure other smallholders will have breeds to suggest.
AI should not be a problem if your cows are well handled. it will also be easier for you to spot when they are in bulling if you know them well. All that would be required is some form of restraint in the barn, a halter would be adequate.Obviously some practice before hand would be good.
Also consider your fencing. Barbed wire on top of existing fences might be needed.