Kylox's Voltstrider

Collect evidence 6: This Vehicle becomes an artifact creature until end of turn.
Whenever this Vehicle attacks, you may cast an instant or sorcery spell from among cards exiled with it. If that spell would be put into a graveyard, put it on the bottom of its owner's library instead.
Crew 2

Murders at Karlov Manor

Collect evidence 6: Kylox's Voltstrider becomes an artifact creature until end of turn.
Whenever Kylox's Voltstrider attacks, you may cast an instant or sorcery spell from among cards exiled with it. If that spell would be put into a graveyard, put it on the bottom of its owner's library instead.
Crew 2

Murders at Karlov Manor Promos

If you can't exile enough cards to meet or exceed the required mana value, you can't choose to collect evidence at all.

Once you've announced that you're casting a spell, players can't take actions until you've finished doing so. Notably, opponents can't try to remove cards from your graveyard to stop you from collecting evidence.

You choose whether or not to cast a spell from among the exiled cards as Kylox's Voltstrider's triggered ability resolves. If you do, you do so as part of the resolution of that ability. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.