Mechanically, start from what "research use only" actually means, because most of the downstream questions are answered by it. It means no release testing, no pharmacovigilance, no regulatory obligation to you, and no recourse.
A defensible group-buy structure has three properties: the material is tested before it is split, the test is paid for from the pool rather than by the organiser, and the split is documented with photographs and a per-participant record of lot, volume and date. If any participant can reconstruct what they received from the records, a later dispute is resolvable. If not, it is not.
Stated carefully, on declarations: the accurate description of a research reference material is a research reference material, and accuracy is both the legal position and the practical one. A declaration that misdescribes the contents converts a customs question into a different kind of question, and it does so on a document with your name on it.
Independent testing costs have been stable enough over the past two years that amortisation arithmetic across a lot is worth doing before choosing a lot size, and the numbers usually favour a larger lot tested once over several small lots tested never.
Test the first lot from any new supplier, set your accept threshold before the result arrives, and keep the certificate with the lot number and the date in one place.