Is it possible to solo mine a block with a single newpac?
Technically - yes.
Realistically - bloody unlikely even over several lifetimes. Should you buy 1 with the hopes it will find a block, maybe if it cost say $20 or so, sure. That said, considering they cost next to nothing to run and you have a computer that can be on 24x7x365 for it to be plugged into, go for it.