These tracks have rain - Spa could have rain Gran Turismo 7 wet weather: Which tracks have rain? (racinggames.gg)
You will see the clouds rolling in on the weather radar before it actually rains on track so yes it's random but you will still be able to get a gauge on when the rain will hit the track and how heavily it will rain which I think is what your main concern is
Do we each need to purchase IM and W tyres ourselves or does the race setting allow us these if required ?
Purchasing the tyres is the easy part. The challenge is then to learn how to use them in a torrential downpour!
Has anyone seen random custom weather turn into rain, if started with dry conditions (S01). I haven’t, even after multiple attempts in other leagues. I don’t think it’s a thing (or it’s a game bug, gasp shock horror!). I think “random” just means random sky conditions, but remaining dry.