7M-GTE ECU is very hard to chip. You basically have to replace the ECU. That hasn't stopped people from selling chips of course, but they're pretty much limited to doing things like fooling the ECU into timing advance/fuel enrichment for thinking that the engine is still cold and the like.
The best answer that I can give to your question would be to rephrase the question as this:
If I found a Supra that I liked, which was in overall good condition for the money, would a code 51/52 prevent me from buying a car that I'd otherwise jump on?
In which case, for me, the answer would be "No."
I'll qualify that by saying that the idea of re-wiring a knock sensor does not seem to me to be a particularly difficult job, though it would be an annoying one, and I would take the annoyance factor out of what I'd be willing to pay for the car. There's also the possibility that the non-functional knock sensor is hiding a more serious problem, though there would most likely be other symptoms if it had caused damage. I would definitely look close.
The "chipped" ECU is actually something that I'd worry more about.
Still, around here, $1850 for a straight chassis with no problem rust is something that I'd consider buying. If rust is less of a problem where you are, you may have a more abundant supply, making it worth less cash.
