Originally posted by ausmoose A macro lens
A landscape lens (yeah I'm kidding myself these are way too expensive)
A portrait lens
A wildlife lens (kidding myself again since these are SUPER expensive, but it would be nice)
The Sigma 70-300 could be almost everything you want.
Macro, from 200-300mm, for butterflies etc, you have the advantage of focussing from 7 feet away, so you have a chance of not scaring the insect.
It's very strong from 70-about to 150mm, weaker at the long end, but fine if you use it close up. It doesn't really look soft until you start focusing on things a the infinity setting.
Your 18-135 is super at 24 mm... used from 22-40m it should be all you need, for landscape.
Given the cheap part, that's what I'd look for...
A few Sigma 70-300 images.
This image is softer than my Sigma 70 macro, or Tamron 90 macro, but it's like an added feature on a cheap lens so who should complain?
My first few years of digital photography, it was my goto lens, and it does most of what you want. The downside is, it's not a lens you'll use much once you get better glass.
Starting with the 18-135 the logical place to go would be to something like the Sigma or Tamron 70-200 2.8, but those are a lot more money. I've seen Sigma 70-300s for under $100 second hand, and under $200 new.
(A few more images here.. )