Well, you've already got a 16 & an 18, so with a 17, you'll be well on your way to completing the collection.
I love the Idea of centerboards!!! but is the 17 comparable to the 18 at all?
The 17 is not as good as the 18 when it comes to sailing with more than one person. However, the 17 is great for single handing. It is easier to manouver on the beach, handles nicely on the water, and is about the same speed as the 18.
-What does used 17 go for, ballpark?
Really depends on condition. When I bought my first used one 5 years ago, I paid $1K for it (a 1989 I think). It had no trailer, the sail was trashed, and it needed considerable TLC. After all was said and done, I probably ended up spending about $2500 total. Overall the boat did well racing, but I ended up getting a new one. Most used 17s tend to be in the $1500 to $4500 range.
-What inherent problems does the 17 suffer from?
Not much. Dammaged/leaking wing sockets are very common. Loose crossbar end caps which cause the boat to be sloppy. And worn centerboards from being dragged on the beach (they tend to want to project below the surface of the hull).
-What are the most expensive things to replace on a 17?
Get yourself a Hobie catalog. The hulls, sail, and mast are going to be at the top of the list.
-What are the SWEET options (both racing and day sail)
Not much you need to do for one-design racing. A decent set of rudders and an on-the-fly adjustable downhaul system are about it. Otherwise, the boat is good to go out of the box.
-Finally, why does a 17 have the little bow spreader bar on the turbo version?
It's a compression bar. The original 17 was designed with 10 foot long bridle wires for the forestay. Most of the force of these wires is directed upward rather than inward and this allowed the designers to use a lighter-duty construction for the hulls. When they added a jib option, they needed to use much shorter bridles. Using traditional bridle wires would have resulted in a higher inward force than the hulls were designed for, so they use a compression bar to keep the forestay/bridle tension directed upward and not inward.