Echoing the comments that your misgivings about an 'only cow' are correct; she'll be miserable on her own. However, whether she'd be more miserable alone with only a calf at foot for company than being in a herd where she's bullied is a moot point...
I take your point about some dairy cattle being housed full time, Rosemary, but those animals are bred over generations to be adapted to that setup, plus there will be masses of adaptations in the housed environment for their mental and physical comfort. Personally I abhore the idea of a native breed like a Highland other than roaming about on rough grass (or preferably moor) land. One winter, we kept one of our Blue-Greys in the cubicles (which are very up-to-date and extremely comfortable, all the other suckler cows tell me
.) I wouldn't say she was unhappy, but she got very hot and sweaty, and we haven't kept any of them in again. They're very happy roaming the moorland with a bit of hay to help them over the worst of the winter, and lots of natural shelter if they need it. And our bull, a very hardy Angus, is much happier being out with them too. (Although he really loved all the attention he got being indoors for his first winter with us
)
If she does prove to be in calf, and you can and want to save her from a life being bullied, I'd regard it as a stopgap. If you wouldn't have the heart to send her away once she's reared her calf, or be able to rehome her to somewhere that can give her a more normal, natural life, then you may be better to turn away. Quite a difficult position to be put in, however, and I don't think I could castigate you whichever way you went on that.