So the only issue here is 'how easy is it to seal up a keel stepped mast?'
To effectively seal it I think that the best option is a product called 'Spartite'. If the boat you buy does not already have it, it's not too hard to put it on. You basically seal the bottom of the hole through the deck where the mast goes, pour in the Spartite and let it set - similar to epoxy. When you take the mast out of the boat, the Spartite stays on the mast - effectively it is a custom-moulded collar. You still need to cover it with a mast boot.
A little bit of water will come in to your boat through the mast, and run into the bilge. Usually it travels down the inside of the mast, and you will not see it. The actual amount depends on how tight the masthead fittings are, and how many other apertures there are in your mast, and how much it rains. It is rarely significant.
I believe that on smaller boats, the deck-step is preferable, but on large boats I prefer a keel stepped mast, as I think that they are safer. If your rigging lets go for some reason on a deck step - the mast may well go over the side. With a keel step, this is much less likely to happen. I also like the fact that wring for lights and instruments runs into the boat through the mast, rather than through a separate hole in the cabin top. This is just an opinion though.
Unless you are going to be doing long-range cruising, where rig failure is a serious possibility, I don't think that the issue should be a big factor when choosing a boat.