When using a substantive gerundive, what form would a specifying purpose clause take? For instance: "things to be used for fighting," I would use a gerundive (utenda) and then what? A dative present participle? An infinitive? Ut + subjunctive? Is there an idiomatic way of doing this, or other preferred solution? Many thanks.
There are two types of dedicated purpose clauses known to grammarians: ut/ne clauses and relative clauses. I think the latter are ideally suited for the situation you describe. Let's take a somewhat clearer example: “The necessity of sending legates to sue for peace”:
Necessitas legatorum mittendorum (vel legatos mittendi) qui pacem petant.
Note that the subjunctive makes all the difference: It turns an ordinary relative clause into a purpose clause.
One construction in this direction is that used for various commissions:
duoviri viis extra urbem purgandis
tresviri aere argento auro flando feriundo
quattuorviri viis in urbe purgandis
decemviri sacris faciundis
When you have a magistrate for some purpose, using a dative gerundive is an idiomatic way to go about describing their task.