Wow..you do have some challenges.. I think you need to start with creating layers of diffrent plants..so taller ones will provide shelter for some other that are not necessary as hardy. That is only way for you to get some diversity on the hill site.
Start with some trees..what are type of useable trees that do grow in your area in similar conditions?
How about few bay trees...they do survive UK winters of so should be hardy enough over there..they create some windbreaks and are not fussy about soil neither. Trees will slow down soil errosion that will happen a lot in exposed hill sites.
But, it will be slow process..once you get few trees to settle..give them couple of years and start introducing undergrowth..bushes..and again..after couple of years lower growing plants..and so on.
While you permanent planting is getting their feet in..perharps making use of annuals and other seasonal plants will provide enough diversity with flowers and edibles?..you could camouflage some planters by sinking them into ground and piling rocks to hide and anchoring them down for some taller temporary plants.
How about some Grape vines..they are surprisingly adabtable..?..or Kiwis?
When you plant something..keep the medium quite free draining..plain compost would hold too much moisture if you do get heavy downpour..so even it sounds daft..mix plenty of grit and sand into planting holes/containers..you need to keep you plants getting used to harsh conditions..and although they will need little bit of help from some 'good stuff' and perharps mulching, but with too lush planting medium, any new growth will just depend from 'cosy' conditions and you end up being slave for the never ending circle of trying 'provide'. You will need to 'provide' but keeping up with more 'natural' level..or just enough for them to succeed.
Use lot of the natural stones to create planting pockets..and perharps doing lot of cuttings on site will give you plants that are hardy enough , rather than buying plants and planting those..they will take getting used to harsher conditions. Use some of the soil that you have when doing your compost mixes..root will adabt better for surrounding soil.
Good luck with you challence..it will be hard slog..and you will loose a lot of plants to get few to succeed...but after all it will be all worth it..