If it really only has about 15 dives and it was cleaned well after salt water use, chances are very good that it's fine as is. But you do need to go through a pre-dive check. You can look up couv's excellent post on inspecting regulators. Here's a bumper sticker version: 1. Make sure the dust cap is in place, then try to draw air on both 2nd stages. You should not be able to. 2. Put it on a tank, pressurize, immerse everything in a tub of water, look for bubbles. There should not be any. 3. Find an IP gauge that fits on the end of the LP inflator hose. Pressurize the reg and see if the IP is stable between 125 and 150 PSI. It should not creep up.
If all of those things check out I'd dive it without any hesitation. $180 is a good price. The MK25 is one of the best 1st stages available, and the S555 is a good 2nd stage, not exactly my style, but I'm weird like that. I don't know much about the particular titan you mentioned as an octo, but I would bet it's perfectly fine.