Go for it!!! Cattle are great!
We have a few Devons, similar to the Sussex - And they're wintering outside with minimal feed/shelter... Lovely animals, looking to take on more land so we can get more!
It depends what you want long term really - A couple of steers would be good if you were wanting to take on and finish them for the freezer or a boxed beef type scheme. If you were wanting to look a bit longer term (though on 4 acres may struggle for space) you may think about a couple of in calf heifers, or a cow with calf at foot. However, you'll have to consider getting back in calf again - AI, use of neighbours bull!
Make sure registered land for keeping cattle, herd number etc and know where you are for TB testing purposes. If you get Sussex, you'll probably be ok without handling facilities, we TB test ours just pushed up behind a gate, very quiet and well handled.
Best bit is probably to talk with your neighbours with the Sussex - Maybe a couple of steers would be a good start for you to get to grips with them without a long term commitment - Glean as much advice you can from them. Beware prices are quite keen at the moment for all ages of cattle!