Eric's 125 build.

[Srt4eric]

not using your existing live sand should still be good..Or mixing for deeper bed?

I had no way to keep all of my sand warm enough long enough. I do have a little over two inches of the old stuff in a 10 gallon with a heater and hob filter

 

[H2OPlayar]

I would bet that there is a lot of good bacteria that can last cold. But that is just a guess. Since you will likely have a mini cycle anyway, any reason not to use old sand other than worrying about spiking nutrients with live die off? Kind of the same strategy I am thinking for when the live rock shows up, throw it in a big enough system and let the good bacteria work.