Hi Miles and welcome to the board. You will learn much from many here who have knowledge and gladly help others.
We need to know a little more about your proposed engines. What spec 454 are the base engines? Do you know what work was performed on the 454's to "Mercury racing specs". That phrase doesn't mean much without specifics. Those engines came in various stages of performance. The base engines were 330 hp with cast internals. There were also 420 hp versions that contained forged internals. These engines were very reliable power in their categories with the 420 hp engines being able to create much more power with simple modifications. TRS drives are now outdated, however, they were virtually bullet proof in that power range. Before purchasing the boat, you should have a qualified surveyor survey the boat and driveline. There are many names that can be recommended should you choose to perform a survey on the boat. If everything is in good condition, that is a very nice older boat to have. Where are you located? If you post some pictures of the boat, someone here on the board may have some useful information to share. Good luck.