Hi,
I don't think you'd gain much from the 18-200 if you already have the 18-135, unless you find yourself needing a bit of extra reach.
You might want to consider the 55-250, nice lens at a great price if your on a budget. Also the 70-200L F4, great lens at a fantastic price.
I suspect you'll get mixed comments about UV filters etc. If you are going to leave it on for protection, get a good quality one, no sense paying for a great lens and sticking cheap glass on the front.
I'd use a lens hood all of the time, reduces flare, increases contrast and protects the front of the lens.
I think I have a UV filter on my 'standard' lens for protection but nothing on my 'good' lenses, looking for best IQ.
You might want to consider Sigma 10-20 for your landscape stuff.
You could then have 10-20, 50, 18-135 and 70-200. A lens for every occasion!!
Cheers,
Kenny