A late 80's Raleigh Marathon is a nice bike to have. It was second from the top of the line in the sport touring category. Those old sport touring bikes are the best commuters ever.

You don't need to be in a hurry to change the wheels to 700c. Especially if you already replaced the original wheels with new 27 inch ones. You have a lot smaller selection of tire sizes in 27 inch compared to 700c, but the ubiquitous 27 x 1 1/4 tire size is the width you would want anyway.

I have a mid 1980's Raleigh Pursuit. The frame is of similar construction to the Marathon. It came with lower quality components. It has some decent Weinneman single wall alloy rims. Someday I will replace them with 700c double walled rims, but I'm not in a hurry. I swapped out the center pull brakes for Tektro R556 extra long reach double pivot side pulls. These brakes were a nice upgrade and have enough reach for when I eventually switch to 700c wheels.

In summary, when I need new wheels, I'll buy 700c's. Until then I'm perfectly happy with the 27 inch Weinneman's. Also, I'm glad to have an old sport touring bike that can take 32mm wide tires.