I would save up $200 more dollars (for the pair) and get the Peavey SP2's instead of the SP5's. You'll outgrow the SP5's pretty quick. The SP2's also have Black Widow woofers in them instead of Scorpions. I really like the Black Widows. Both have field replaceable baskets. This means if you blow the woofer you can replace the basket (everything but the big round magnet) for about $35 instead of $150.
I don't know anything about Samson amps. Make sure you understand the power you are quoted. You'll get the most out of the amp in bridged mode (single output channel) but you're usually limited to a minimum of 4 ohms load. Some will state 2 ohms but not for continuous use. I think the Peavey CS series amps are rated 2 ohms continuous. Anyhow, your amp will be spec'd for bridge, 8 ohm, 4 ohm and possible 2 ohm load. If you run a pair of SP5's or SP2's your load is 4 ohms bridged or 8 ohms if running channel A and B on the amp seperate. I use Peavey PV900, PV1500 and PV2600 amps and love them.
Outside use? Hmmmmm. Playing outside you'll lose a lot of your bass. The only solution is to add subs and a hefty amp to push them. You'll also need a crossover (active) to split the signal out to your mains and subs. Once you get to this point you'll be bi-amping your mains (SP2's). This means you'll have one amp that feeds the SP2 horns (highs) and one amp to feed the woofers (mids). The signal will come from your mixer (main out) to the crossover (where it will split 3 ways) then to your subs, horns and mids.
Mixers? I really don't know anything about the Yamaha mixers. You could go cheap with a Behringer and hope it can take the abuse. Or look at some of the Peavey mixers. The Peavey mixers are probably about the same price range as the Yamahas. The Peavey's seem to have a good reputation for holding together in live use. I started out with an old tank from a pawn shop and bought a new 20 channel Peavey several months back. No complaints.
Sorry I got so long. Hope this helped.