Lens Set for 500D
For landscapes and architecture I’d seriously recommend going wider than 18 (28mm equivalent).
24mm (Full frame Equivalent) is a really nice focal length for these subject whether a landscape where you want the panorama or a confined streets where its often not possible to move back to fit the subject in.
This then leads to what can be used for APS-C to give 24mm (Full frame Equivalent) which is 15mm ? When I was in this situation I considered a medium priced prime (but none are available: 14mm L, 15mm Fisheye, 17mm Tilt shift L) so your only choice is the Canon 10-22 or the Sigma 10-20 and the Canon 10-22 is expensive.
For longer reach I’d agree with the previous post the 55-250 is good value, but the Canon EF 70-200mm f/4.0 L gives excellent image quality and is about half the price of the Canon EF 70-200mm f/4.0 L IS version. The image stabilisation is very nice but you can compensate for not having it with a tripod (The Joby Gorillapod GP3 is small & light 241g and really useful in the city), you won’t be able to compensate for a poor optical quality though if you get a cheaper lens !
In my experience, and in that of many others, you will tend to use mainly just one lens about 90% of the time so you could try the Canon EF-S 17-85mm but it does suffer from distortion and Chromatic Aberration at the wide end (and it’s not wide enough). Don’t be tempted by the 18-200 super zooms, their continent but you lose in image quality.
An alternative to the Expensive Canon EF-S 17-55mm f/2.8 IS is the Tamron 17-50mm f/2.8. Ok it’s not got Image stabilisation but its half the price and gives comparable image quality.
But like I said initially seriously consider trying wider than the 28mm of the kit lens (once you’ve tried it you won’t go back.)
Hope that of some help ...