+1 on what KVE77 said.. - The days of needing a 100w Marshall & two 412s has long gone (thank god!)
Small valve (tube) amps miked up for electric guitar, take a line out from the bass and plumb into the PA, mike the kit and any other instruments such as brass. I'd take a line wherever possible from acoustic instruments such as guitars, banjos and mandolins assuming they have some form of internal pick-up.
I've used many systems over the years (I've been an audio engineer for more than 15 years) and all the mentioned above are on the money. The best I've used are without doubt EAW KF series speakers ran with Lab Gruppen amps and an Ashly Protea controller but thats big £££ although EAW do smaller systems too. L'Acoustics are excellent (their ARCS series horn is clever) as are Turbo Sound but again, thats all big £££. Bang for buck, I'd go with Yamaha or Electrovoice as their products are usually superb as long as you stay away from the low budget ranges although to be fair, even they're largely ok. A local venue has recently had a refit with soome active EV enclosures and it sounds pretty sweet..
Consider you may need stage monitors, microphones, stands and of course a mixer (i'd go digital) too!
Lastly it might seem madness to mike everything up - it's not to make everything louder despite what some think - it's to give much more control over all the sounds and make the final mix a far more believable sonic picture of what the musicians are creating. This is especially true in an out-door scenario. Plus the on-stage sound will be far more balanced which will keep them happier as anything plugged into the system can be sent to an onstage monitor speaker..
So to sum up..
Speakers: Yamaha, EV, JBL, Turbo, EAW, Funktion One, L'Acoustics etc..
Amps: Crown, Crest, QSC, Lab Gruppen, Yamaha, FFA, MC2..
Mixer: Yamaha, Allen & Heath, Soundcraft, Mackie are the usual suspects..
As ever buy the best you can afford!