Yoga Sutra 1.14
स तु दीर्घकालनैरन्तर्यसत्कारासेवितो दृढभूमिः॥१४॥
Sa tu dīrghakālanairantaryasatkārāsevito dṛḍhabhūmiḥ||14||
Long, uninterrupted, alert practice is the firm foundation for stabilising the fluctuating mind.

Minimum of 2 years of consistent Iyengar Yoga practice required. You must be ready to practice Sarvangasana and Sirsasana (or know their substitutes). You also must know how to take care of yourself.
Minimum prop requirement: 1 chair, 1 bolster, 3 blankets, 2 cork or wooden blocks (foam is too unstable) and a belt
It would be very helpful to also have: a second chair (it does not have to be a yoga chair), a second bolster, 3 extra blankets and 1 extra belt