The K-5 is everything the K20D is not
Try one and it will leave you giddy with excitement.
As for ISO6400(in RAW), the K-5 performs like the K20D does at ISO2500.
As for the triggers, the best low cost alternatives I've found next to the Pocket Wizards or Flash Wave models were the infamous Cactus flash systems by Gadget Infinity. Which have received a substantial amount of video reviews on Youtube etc. so you might want to check those out when you get the chance.
Anyways, The K-5 low light performance is top notch and bar none the best Pentax has ever had. My advice to you would be to head out to your local store and try one out for yourself.
PS. bring a blank card with you