The classical era sources are very limited. Take a look at Caesar's 'Spanish War' (especially the section on the battle of Llerda) and Strabo's survey of the entire known world.
Here's what he has to say about Lusitanians:
" --- the Lusitanians, it is said, are given to laying ambush, given to spying out, are quick, nimble, and good at deploying troops. They have a small shield two feet in diameter, concave in front, and suspended from the shoulder by means of thongs (for it has neither arm-rings nor handles). Besides these shields they have a dirk or a butcher's-knife. Most of them wear linen cuirasses; a few wear chain-wrought cuirasses and helmets with three crests, but the rest wear helmets made of sinews. The foot-soldiers wear greaves also, and each soldier has several javelins; and some also make use of spears, and the spears have bronze heads.
All the men dress in black, for the most part in coarse cloaks, in which they sleep, on their beds of litter.
All the mountaineers --- let their hair stream down in thick masses after the manner of women, though before going into battle they bind their hair about the forehead.
The Iberians were once, virtually all of them, peltasts, and wore light armour on account of their brigand life (as I said of the Lusitanians), using javelin, sling, and dirk."
I'd use figures listed as 'Spanish' generally, with some Celtiberians added in. No scutarii, but find some larger rimless round shields to mix into the infantry formations (I used some meant for Foundry Arthurians).
You could also make the legionaries different by having a proportion (who knows how many?) without mail, and equipped with a handful of havelins rather than pila. Read the account of Llerda to see the problem's Caesar's force had with an army of (Pompeian)Romans who had long been in Spain and adopted local methods.
