Hi Pinno & welcome to TAS
Do you plan to start breeding straight away ? 'Sow' normally refers to a breeding pig after she's had her first litter and if you mean young females it's normally 'gilt'.
I would really suggest you start by raising a few batches of weaners for the freezer first, for a number of reasons...
1) You'll get to 'practice' your husbandry skills (jagging, tagging, loading) and build your confidence
2) You can try out all the different breeds to see which suits you best in terms of temprament and style/ease of management and how they impact on your ground, fences, lifestyle

.
And lastly,
3) You'll only need the one house cos weaners will usually all sleep together happily
The other thing to consider, depending on where in the UK you are is how available and accesible the breeds are that you fancy actually are. In terms of 'most rare' you're looking at British Lop, Middlewhite, Berkshire, Large Black and Large White's (think probably in that order too)
HTH
Karen