This is a HUGE topic, and I would say get a book or two by Nigel Calder and start reading. You'll get at least as many opinions here as there are sailors on this board. One point you might not appreciate is that you should not combine old and new batteries in one bank, so it wouldn't be a good idea to simply add one new 27 to your two existing ones to form a house bank. Beyond that you need to start by adding up your daily power needs, and only after that will you be in a position to evaluate battery and alternator capacity.
In our case we had two group 31, 130 AH batteries as our house bank that we also used for engine starting, with a separate engine back up battery just in case. But we never ran our fridge, and didn't have RADAR, so our power needs were quite modest, so the existing alternator worked for us.