I'm not sure why you're nervous about a higher airframe time. I've seen some low airframe time planes that were junk and some really good higher time planes. Have a very knowledgeable mechanic (knowledgeable about Cessnas) do a THOROUGH pre-buy (my preference is have him annual it, then his signature is going in the books.... and you should be good.
The plane you referenced there is on EDO 2790 amphibious floats. Those floats can work, but your gross weight will be restricted, and you'll really have to learn to fly those floats. My guess is that at some point, you'll get into the habit of overloading it a bit, get in some wind, turn it around and have a deeply moving religious experience

. I've seen it done. Those floats are significantly under floated for a 185, in my opinion. Also, FYI, all the float compartments from the step forward are connected, so if you hole one compartment, instead of flooding just that one compartment, you'll flood the entire front of that float. Unless you're toed up on land on a sloping beach, you'll lose the plane. I know....don't hole a float. I've done it...more than once. It happens.
If you're looking at a plane that isn't already on floats, make certain that it has a factory float kit. You DO NOT want to consider installing a float kit.
As to Wip 3450 floats rigged for a 206 and re-rigging to 185, bring $$$$, lots of it. Also, in my opinion, unless you go to the upgross on the 185, the 3450 is too big a float for the 185. If you're going with Wips, the 3000 is a great float for the standard 185.
I agree that the EDO 2790 floats are a poor choice for the 185. I think that, properly maintained, and used in a recreational airplane, the Wip floats, the PK floats and the Aerocets are all good choices. The Aerocets are great floats, and fiberglass has it's advantages, no doubt, and not just in salt water. That said, these are very large floats, so cruise speed is slower than the smaller floats.
I REALLY like the IO 550 engine, but the 520, operated intelligently, works fine, and at overhaul you could exchange it for a 550. I think you're going to find that 185s with 550s are going to be pricey.
Wing extensions are a GREAT seaplane mod on the Cessnas. On many models they give you an increase in gross weight. With some, the 185 can go to 3500 pounds.
Finally, you may want to consider a Cessna 180 with a bigger engine and upgross mods. They're a little lighter, and with a 520 or 550, they make great seaplanes. There are probably more 180s out there that haven't been thrashed in commercial use as well. In any case, I wouldn't exclude a good repowered 180. I think for the money you're talking, you could get a really nice airplane with a very low time engine.
MTV