400.00? That seems a bit high. The questions are:
What does the revell kit go for compared to the Tamiya and is there enough of a difference to matter?
What P.E. set are you thinking about and is there another one on the market that would work out and maybe be a bit cheaper?
What about wooden decks and brass barrels?
What's the going price of paint and thinner and how much will you need?
What kind of diorama are you planning and what materials will you need for that?
Maybe you could build the ship and do the diorama at a later date.
I have a model of a very well known sub that would look good in a number of different dioramas but that will have to wait for some time.
What tools do you need to replace and at this point do they need replacing?