Lens for shooting paintings with Canon 550d

You'd be best off getting a medium telephoto to minimise distortion - upwards of 80mm I'd say, with a cropped sensor camera. Sigma make an excellent, sharp 105mm lens, the 105mm EX-DG Macro f/2.8, which is about £350-£400.

You'd need to set up the camera a fair way back from the photo, but should get good results with this sort of focal length. Make sure you stop down as much as is practical to ensure as much of the painting is in focus as possible.

I think a 50mm 1.8 would be ok for your work, considering that you have pieces that are up to 3m long you will need a long room to fit the painting with a 105mm lens. Also the 50mm is cheaper, sharp and you will have enough money left to invest in color management to render the colors of the artworks as accurate as possible, or, buy some lights.

For color management, I use RAW-Processing Accelerator, and for monitor calibration I use spyder cube which will help you a lot with the processing.

It's hart to get a good lens in the $300 price range. You can have good results with the 18-135 EF-S lens as long as you a have enough light to shoot at F8 and enough distance to have your focal length between 70 mm and 105 mm. This is your best option on this budget.

When photographing artworks for sale, I use Canon's 24-70 f/2.8, which is way out of your budget; However, the reasion I do this is that with a compatible camera body, it stores a lens profile in the image produced that can correct for distortions during post-processing. A list of currently supported lenses is available from the Adobe website

You don't need a lens that expensive though; If you use the RAW mode on your camera and use Adobe Lightroom, then you can correct most distortions by hand, if a lens profile is not available. In which case, I'd probably look to recommend a lens that is going to be flexible and allow you to work in tight situations, such as in the artist's studio. You won't be needing a particularly wide aperture/narrow depth of field for this (otherwise the 50 f/1.8 is one of the best value for money Canon lenses), but you might need some fairly wide angles for some of the larger pieces, if you are in confined spaces, in which case, I'd probably err towards the 18-135 that you're already considering - it might mean a little more work getting the post-processing sorted, but it is a good all range for use as a general purpose lens.