I was thinking of purchasing a small Avocado plant but,I am unsure what sort of compost to put it in to re pot and what about the winter will it survive our cold winters?
NO. It's a warm weather plant from tropical and sub tropical areas. They don't like frosts.
Thanks for that but , would it be ok in the greenhouse over winter or indoors
Depends on how big your frost free greenhouse is. If its the size of the one at Kew, yes,.
Avocado trees get really big.
Don't purchase one. Grow one from pit