PA for FOH and another for foldback (monitoring...), unless you're going 'In Ear Monitoring', which is a whole art in itself. 'Stereo' is not essential, but useful. Rough guide..? 2 x 500-1000w for FOH, n x 200-500w for monitoring. How many Aux channels on the mixer..? More is better, to enable a mix for the singer and another for the rest; sometimes a mix each is asked for, in which case a digital mixer and powered monitors would fit the bill.
All-new gear can be expensive; s/h might be worth a look if some bands sell up, given the current climate for gigging. What makes..? We have been using HK for some years and find it worth its price, but we're not raucously loud. Many swear by RCF. We don't use subs (FOH for voice, mostly, and acoustic guitar, some keys; no bass in FOH...). Some need subs, though; they need more power and take up space in the venue, in the van and in storage. It's a choice.
Other than that, what's your budget..? That'll dictate much. Hope this helps.