As Devon has two superb native breeds, I'd say it would be a great shame not to use one of them! Both are recognised as producing good beef, both will attract the native breeds premium. North Devon aka Red Devon / Red Ruby is a very hardy animal, produces marbled beef on grass, mothers are good mums, docile and milky (it used to be a triple-purpose breed), the only concern would be that they may slab on too much fat on good South Devon grassland! (Mind, Belties would be at least as bad if not worse on that score.)
I have no personal experience of the South Devon but we are starting to see some up here in Cumbria, amongst the Limousin herds. Bigger pelvises, calmer cows, grow well on less cake, is what people tell me they like about them compared to the Limousins.
If your requirements are 'pure breed, can convert forage to premium beef with minimum fuss / housing / supplements, marketable, hardy / can outwinter' in Devon, then personally I'd be going straight for the gorgeous Red Rubies.