I figure you'd go very far with an ND filter Vinze, considering the type of shots you take, and you've got the patience. Tripod should be next on your list as well.
On top of 'balancing' the color tones out, ND filters reduce light entering the lens. This may seem like a bad thing, but some shots call for it. Skyguy's example of a bright sunny day is one. Secondly, your FZ18 only goes down to F8... that's not very small. I'm sure by now you've seen those 'flowy' waterfall pictures, takes about 8 seconds to get a good one. If you're there on a sunny day, you're probably going to shoot at F8 with 1/15 shutter and its still over exposed. What do you do? Stack up 2 ND8 filters for a ND16.. (exaggerated), and you can drag the shutter speed for the flowy effect.
Alternatively, long shutter speed photography has its uniqueness. It eliminates transient objects. Say you want to take a picture of Orchard road's landscape, but everyone keeps walking past your camera! Stack up your ND filters, expose for 30 seconds... you realize all cars + humans will 'disappear' from your picture. =)
Unfortunately, I for one do not have the patience for this.
UV is highly recommended... I bought two new lenses last saturday, guess what, one of my girl-friends handled my camera for awhile, when I went home, I found a finger print smudge and 10 scratches (probably my fault too due to the lens cap). And its a Hoya dual-side coated filter somemore! apparently the glass(plastic) is not scratched but the coating got scratched off. And this is only 3 hours after the lens came out of the box. Imagine the sorrow I was drowned in! Fortunately it wasn't the lens itself or I'd have really done something pretty silly. After that moment, I decided that all my lenses should have hoods + filter.
Contraversially, there are those who disagree with filters. Their reasoning is pretty logical too. Lenses these days are made of strong glass and are not subjected to user marks that easily. Neither do you want to have high quality glass obstructed by a cheap front objective... Imagine always shooting through a window...Filters also increase the chances of flaring. Some more than others.
Your best bet is a Olympus TCON 17 (~135 brand new - should be TK photo) followed by Raynox 2020/5 ($380 brand new - McJill's shop). All, make sure you really need the range before getting. That includes doing things like birding, and candid sniping. Quality loss is noticable when using the TCON, but controlled, pretty good in my opinion.
Besides a TCON, the only other conversion worth getting is either a raynox macro or close up filter.
I disagreee!! I wouldn't buy digital king for sure... Neither will I buy Vitacon because I had a CRAP! experience with it... Although Skyguy has one copy that seems decent... Eitherways I say 30-50 is a more accurate price.
All in all, especially after upgrading to dslr did I realize the importance of optical quality. The Leica lens' wide angle is quite exceptional already. (Notice there is no barrel distortion!!!! Something I have to live without on my dSLR) Put on a wide angle adaptor and all the distortions will come in.
All that said, I'd like to share my shot of the day, really love it.