well with only 6K that kinda limits you.
1)I think you should buy the highest efficiency boiler you can afford. I recomend buderus.
2) what kinda tank ? an inderect fired hotwater tank is a must
3) what exactly are you trying to do ? there is a lot more to it then just pick a size also concrete is the worst medium for radiant are you adding this in an addition ? how were the rooms heated before hand ? maybe forego the radiant now and buy a better boiler