One cranking and a deep cycle bank large enough for your needs makes sense. The 6 volt batteries I suggested are the most durable and the best bang for the dollar and 2 in series will give you about 225 AH @ 12 volts.
No manual for the alternator? If you google the numbers stamped of it you should be able to find its output. Failing that a clamp ammeter on the positive output with the engine running and the batteries in need of a charge will give you actual output.
Isn't it always worth considering a dual purpose or deep cycle for the 'starting' battery? So long as it can put out the cranking amps necessary it seems better to have that reserve available. It seems unlikely that the additional cranking amps from a pure starter are necessary.