Loads of advise here Tas, so I will keep my twopenneth short.
At 100 hrs a year a group ownership of some sort, in my humble, would save you, ballpark, £50hr over hiring.
Key elemnts are as mentioned - check out a prospective group well, especially the finances. We work on £60 and hr and £64pm for a pa28 161 with ten in the group. A good reserve and money in an engine fund from every hr at the moment. This is a fluid amount and assessed 3 monthly.
Overall though, if you get a good group, you will be able to expand your flying no end. In the first year I have near enough paid for my share in the savings I have made.
Good luck Tas
LF