It's hard to say, if the issue was diagnosed and verified as a cylinder head at 134k, it's most likely that it will rear it's head up again. When exactly is hard to say for sure. Removing the "gunk" is going to speed the process along because that gunk is what was sealing the head up before hand. I wouldn't be overly afraid of the engine giving out on you right now, but realistically long trips will most likely be the place it will happen if it does.
Since this dealership seems to at least be working with you and trying to do the right thing I don't think you need to jump all over them, but stand firm on your position and concerns.
i.e.
-Firmly, but amicably present your concerns with the headgasket & vehicle reliability and ask for the dealer to either (a.) provide an extended warranty on the engine (or maybe just an extended warranty package? I dunno what they could offer you) under their cost (b.) replace the headgaskets and repair the engine under their cost
-If they refuse to cover the cost 100% on either of those two options, remain courteous and reiterate that when you purchased the vehicle it was under the assumption that it had been inspected, the history ran, and all potentially large issues such as this had been diagnosed, documented, and/or repaired. While these things can happen, it is still unfair of you as the new owner to be saddled solely with the burden of a potentially unreliable engine and costly repair bill due to an issue that was not caught when the vehicle was traded in, inspected, and placed on the lot. As such you'd at least appreciate that they compensate you partially and work out an acceptable amount of assistance for the cost of either (a.) an extended warranty (b.) the headgasket repair itself.
-If they continue to refuse and begin to stonewall you on the subject matter, I would take the research on Lemon Laws and your legal rights and options in this situation and calmly present them to the dealership. Again stating your concerns and dissatisfaction with the vehicle being presented in an inaccurate manner (again, not entirely their fault but at least the signs of a larger issues should have been noticed when the vehicle history was run and inspected.) State that you feel it is unacceptable for you to feel afraid that your vehicle's engine can give out at any given time because of this previously unnoticed problem and that shouldering the cost and responsibility when you were given the assumption of a fairly problem free vehicle when purchasing it is not only unjust, but somewhat dishonest in your eyes. You do not want the vehicle as it is not the vehicle that was presented to you when you agreed to purchase the vehicle and they should accept the return of the vehicle from your ownership. Failure to do so will unfortunately result in you having to follow the correct legal recourse in this matter as well as contacting your regional Toyota/Lexus Office and having to inform them of this situation as well as any other organizations that are involved in these matters.
The basic idea hear is that since they are being helpful right now, there's no reason to be aggressive or demanding. But things can always change when it comes down to "who foots the bill" and to make sure you are prepared for all outcomes, the key is remaining firm on what your realistic expectations are and that any less is not doing right by you, the customer. If they pull a 180 on you and go full "not our fault, you bought it, you got it," you will most likely have to go through legal means to correct the situation.
Basically at this point if you haven't already, record & duplicate everything in your conversations and interactions with all dealer personnel in regards to your vehicle and the service you are given. Also research your applicable Lemon Laws and legal options should things go south (in case they do try turn on you, you're prepared and ready) as well as at least have contact information for a good attorney if need be.
It's an unfortunate situation and I wish you the best in it, but the fact of the matter is you were unwittingly wronged by this dealership (who they themselves were duped by the original owner of the vehicle). While they are a business and must turn a profit, they need to do the right thing and take care of the customer due to this unfortunate mistake on their part, at the very least they need to meet you half way on getting the vehicle back to or better than the condition it was presented to you in at the time of sale.