Blown headgasket, replacing engine assembly

I recently bought a 2011 prius that had 300k miles on it. Shortly noticed engine shaking and loud noise, p300 to p304 came up, as it also consuming coolant, I assume the coolant is leaking inside as there's no visual leak. I'm gonna go ahead and swap the whole engine. Found one at LKQ that has 91k on it for 1100$, my mechanic would do it for 500 which totals 1600. Should I change the transaxle assembly too? No codes related to it for now but I'm just afraid the loud noise comes from there too, if the transaxle is bad, will I get codes for it? I'm more inclined to changing it since it has 300k miles on it and it's just gonna cost me like 500$ more( 61k miles transaxle at junkyard for 250$) What are your opinions? Thanks