It will require a more deep analysis, but I guess there is always the option to organize a BS group buy for performance items (i.e. brakes, suspension, etc) which means to collect money from members for a certain specific part.
However, In my opinion, the Consortium should be restricted to Q4 parts plus other parts that Q4s have in common with other 155s which are difficult to find (mirrors? etc).
In any case, believe that to assign kind of a membership will be a good step to benefit those who have commitment to support the project. Membership lets say EUR100 /150 (?) non refundable
Parts to be offered among members at cost price or to other BS participants with a mark up (in case no members are seeking for that part)
Not easy to decide though how to manage a situation in which 2 or more members require a part which we have just one in stock....in favour of the oldest one?