Hi Sarah,
Being a light skipper means you have to choose a boat you can sail easily, since you`re looking at single-handers, just a few points :
-You must be able to get the boat on & off a trailer, rig and de-rig, and get it to the water and back again, sometimes alone if there is no-one else to help.
- You MUST be able to right the boat alone, and preferably without the use of any aide other than a righting line. Why ? I don`t think a righting pole, bag or any other device that adds a step to righting the boat can be safely used when it really counts - all the demonstrations of these devices are done in calm water in light wind, when you are unlikely to capsize anyway. Try using a righting bag in strong wind in big swell, when you are doing your best just to keep your feet on the boat, now fill a bag with water and sling it over your shoulder (I have, on the Dart 18, it doesn`t work). Now capsize close to rocks or the surf line, or in a shipping lane just for interest. Or try standing on a pole in the same conditions. You have to be able to right a boat QUICKLY at all times.
- You must be at or near the competitive weight range of the boat in most conditions. Why ? You might win a few light-wind races but it`s no fun struggling to depower the boat while your opposition sails past you in over 10knots.
- You must be able to comfortably handle the loads on mainsheet, downhaul, spinnaker etc, or you will just exhaust yourself rapidly trying to keep up with the rest, and your performance will drop off toward the end of a regatta, unless you are very strong and fit for your weight.

I don`t know the FX or Inter 17, so can`t comment, but I believe they are both the same weight (roughly) as a Dart 18. Unless their masts are much lighter than the Dart, you won`t right these boats alone. I also believe both these boats have fairly stiff masts which will increase the loads on mainsheet & downhaul, and you might struggle to depower the rig a bit more than you would on a boat with a more flexible, lighter mast.
You do need to choose a boat that you can race in a class, though. It won`t help to sail a boat that no-one else has unless you are ok with sailing in an open class if your club supports one. If you choose F16 you will be the first one, but others might follow and a class could grow, but it`s always easier to go with the current and join an existing class. I did that for two years, and sailed a boat that was totally unsuitable for my style of sailing (I like adjustable, tunable rigs), so I really wasn`t enjoying myself. I then switched to a boat that allows fully adjustable everything, and I won`t look back.
What I`m saying is that if you enjoy sailing the boat, you will enjoy racing it, but if you don`t enjoy sailing it, chances are you will struggle to do well, and will hate racing it.
Just my opinion.

Steve