I don't think you'll be happy with this:
http://www.musiciansfriend.com/product/Peavey-Messenger-Portable-Sound-System?sku=601060
You want a solution that you can add on to, rather than replace, having wasted your $400.
Try buying just one of the powered 10" JBL EONs or
Mackie SRM 350 and a used cheapo mixer like a behringer or something. That's a bit more than $400, but only a little, and the difference will be somthing akin to getting a real car instead of a go-kart.
$550 powered speaker, available anywhere-
http://www.music123.com/Mackie-SRM350-Powered-Speaker-i120708.music
I have
the Mackie SRM 450's and they seem to be the gold standard around here for small/ medium venue gigging musicians. The 350's should be great for vocals plus acoustic if you can swing one.
Or...here you go for $350:
http://www.music123.com/Peavey-112PM-Floor-Monitor-i48747.music just add a used mixer ($50-100 if you know what to look for) but trust me, scrounge some change for the Mackie...
$60 Mixer:
http://www.musiciansfriend.com/product/Behringer-Xenyx-1002?sku=631264X (ok, they're out of them now, but you see the kind of thing you can get for peanuts.) Behringer sucks, but better to get a nice speaker and upgrade your mixer later.
$200 mixer (this is what my band uses. Sounds great. Built well. Can handle a 5 piece jazz band and vocals)
http://www.musiciansfriend.com/product/Mackie-1202VLZ-PRO-Micro-Mixer?sku=634219
At any rate, the one powered speaker route is the way to go, since you can always use it later as a monitor in *any* system, or throw out your cheapie mixer and get a nice one with nice sounding mic preamps for your vocals and sound even better. Save the $$ from passing the hat and get another powered speaker and stands for better coverage and fuller bass...
Good luck!
Oh, and for reverb, find a used alesis nanoverb.I see them for $30-$40. If you can't find one, PM me and I'll sell ya mine!
Joe