Txp should be required (or otherwise) by Xmas. Why do you say April Chilli, I thought proposed mandatory Mode S was still a few years off?
Not sure about fitting a 2nd-hand Txp without the instruction manual (and who keeps them!)- is there much to it? Does the altitude encoder need calibrating?
Certainly agree with EA that a Txp of some type is preferable, just dont want to make the wrong choice.