Only you can decide what you want in an "outdoor" lens. Take a look at these two sample pages and you will get an idea about the range of various lenses. The crop factor on your Sony a100 is exactly the same as it is on my camera, so pay attention to the numbers outside the parentheses.
http://www.flickr.com/photos/samfeinstein/476181737/
http://www.flickr.com/photos/samfeinstein/1245831147/
As far as reducing haze, there is such a thing as a haze filter which will reduce haze. If I had one on in the second example, the background would have come out better.
But, a polarizer isa great filter for outdoor photography. See: http://www.flickr.com/photos/samfeinstein/2197308037/ and you will want one!
Be sure to get a "circular" polarizer, which is needed for your camera's metering system to function properly. You might consider getting a "thin" polarizer so that you can use it at the widest end of your lens (18mm???) without darkening the very corners of the image.
This is a decent kit: http://www.bhphotovideo.com/c/product/408013-REG/Hoya_GIK55GB_55mm_Introductory_Filter_Kit.html
Quite adequate polarizer for $32.95: http://www.bhphotovideo.com/c/product/22703-REG/Hoya_550194_55mm_Circular_Polarizer_Glass.html
Kickass polarizer with thin frame and a bunch of stuff that you'd never know the filter had or not for $97.50: http://www.bhphotovideo.com/c/product/391180-REG/Hoya_XD55CRPL_55mm_Circular_Polarizing_Pro.html But you can read the "features" list to get an education about filters.
Perfectly adequate UV/Haze filter for $11.95: http://www.bhphotovideo.com/c/product/23716-REG/Hoya_550033_55mm_Ultraviolet_UV_0_Haze.html
Great UV/Haze filter for $25.50: http://www.bhphotovideo.com/c/product/23739-REG/Hoya_558039_55mm_Ultraviolet_UV_0_Super.html
Some people leave a UV filter on the lens at all times to protect the front element. I'm no longer a fan of doing this and I choose to use a plain optical glass filter instead. Here's my spiel on that topic:
Many people use a skylight or UV filter to protect the front glass of the lens, but you can also buy high quality plano glass filters for that purpose. Whatever you do, at least buy a decent quality filter instead of trying to get off cheap.
Personally, I'd say the first place to start for effects is a polarizing filter. You want a "circular" polarizer. Check these examples and you will see what it does:
http://www.flickr.com/photos/samfeinstein/2197308037/
You might also find a neutral density filter (varying degrees of darkness are available) useful for taking shots of bright subjects at wider apertures or slower speeds.
As far as all the rest, you can do this in Photoshop or Photoshop Elements. Okay, I don't want to argue with anyone about how it's better to use the real filter, but our asker can try a few in Photoshop (etc) "for free" and see which ones will be the most useful before you start spending money on the real thing.
Or, go here http://www.thkphoto.com/products/hoya/index.html and see what you can learn.
Or here http://www.tiffen.com/tiffen_filters.html Tiffen makes a kit to get you started on filters, if you think you want more than one. They include a polarizer, a UV filter and a warming filter. B&H Photo and many other places sell this kit.
Many people use a UV filter simply to protect the front element of their lens from damage. "UV filters absorb ultraviolet rays which often make outdoor photos hazy or indistinct." (from: http://www.thkphoto.com/products/hoya/gf-01.html )
We get this question often enough that I decided to upload a sample to Flickr showing the same subject taken with and without a UV filter. Download the image, cut a small section out of the top half and drag it to the same section in the bottom half and see what you think. The photos were taken about 15 seconds apart in subdued sunlight, so I think the lighting was virtually identical for each. There was no post-processing at all so you can make a fair comparison. I will not comment any further and let you decide for yourself if there is any color shift.
http://www.flickr.com/photos/7189769@N04/441244796/
The picture was taken with a Nikon D200 at ISO 100 with the Nikon 18-200 VR lens @ 112 mm at f/5.3.
Here's a comparison that I did by accident. Read the comments and you'll understand:
http://www.flickr.com/photos/samfeinstein/1793551691/
Buy a name brand like Hoya or Tiffen. Don't get cheap junk to put on the front of your fine lens. Personally, I am now switching over to plain high quality optical glass "filters" from Nikon for lens protection, even though they cost more than a good UV filter.