The history of Indian attire dates all the way back to the Indus valley civilization - the Harappan era. Evidence found from iconography shows that men usually wore a long cloth (typically cotton - India was one of the first places where cotton was cultivated) wrapped over their waist like the lower part of a tribal sari or dhoti. Turban was also in custom in some communities as shown by some of the male figurines.

The first century BCE was Mauryandynasty after the fall of the Mauryan dynasty; it was the Sunga period. The most common attire of the people at that time was antariya, which they used to wear as a lower garment. A cloth was covered in lehnga style around the hips to form a tubular skirt. The pleats in the front are called the patka. Ladies wore pretty much the same thing. The upper garment, called uttariya, was a long scarf and the difference existed only in the manner it was worn.

Ornaments latched on to a special place in this era also.

V-shaped sandals were typically worn in that era. However, depictions on some coins show that kings wore full boots.

It's quite interesting that majority of Indians (atleast females) and males in rural areas still wear this sort of an attire carried all the way from near 2500 BCE.

Edit: gramma

Edit2: First centure BCE --> shungas not mauryas

It would somewhat depend on which part of India he was from . (of course, being a long way from home, he might've picked up foreign bits of clothing)
Looking through "Ancient Indian Costume" by Roshen Alkazi:

1. a tourban - This is pretty likely. Men wore tourbans of various kinds through the Mauryan and Sunga periods (called "mauli"), same in the Satavahana period and even later during the Kushan empire (1st c. CE)
2. waistcoat - most common depictions show "uttariya" - a piece of cloth I'd describe as a wide scarf, draped around the upper body, or just worn over the shoulders. There are however depictions of warriors wearing short sleeved tunics.
3. leggings - there are depictions from the appropriate period showing warriors wearing trousers (likely adopted from the Indo-Scythians, or close-living Iranians), though most men are depicted bare legged, wearing an "antariya" - basically a loincloth.
4. boots - many men are barefoot, but some warriors are depicted wearing knee, or mid-calf height boots.
5. arm jewellery - yes. We have depictions of men wearing "baju" bands, "keyura" armlets, or serpentine armbands (which the author claims to be adopted from Iran)

I think the least accurate are the materials - most Indians would've been wearing soft cotton, linen and more rarely silk, not leather.

Followup: do we have any idea how likely it would be for an Indian merchant to be personally in Rome in the first century BC? Obviously large amounts of trade goods came to the city in that period, but my understanding was that many middlemen were involved. Even if an Indian trader sailed his goods to Egypt, do we have any reason to believe any Indians really personally accompanied their goods on the overland journey to the Nile or Alexandria and then across the Mediterranean to sell them in person in Rome? Or is this just an efficient cinematic way of representing the scope of Roman trade that isn't plausible in its details?

Today we think of turbans as Indian.

Was that true 2000 years ago, or could a turban wearer be from somewhere else entirely?