I installed one from Forest Wind, around RM1.4k (can’t remember exactly).
The DC motor died after about 1.5 years, but they honoured the warranty and came to replace the faulty motor.

Not sure if I was just unlucky, they told me it normally lasts around 8 years+.
It does improve ventilation a bit, but honestly, if you have the budget and time, I would recommend spending on proper thermal insulation instead. That gives a much better result overall

30W
yes, roof lining
In that case the solar ventilator should be quite useful
Try to get a model that has a power cable or battery, so it can run at night as well
The heat and humidity trapped in the roof during the day will still remain at night, so you might still feeling uncomfortable at night