If you persist to make wedding shoot just understand that in term of gear a pro would have:
- 2 FF camera with a 24-70 f/2.8 and a 70-200 f/2.8... Lot of margin for low light.
- At least a cobra flash with a big diffuser
- Potentially an assistant to help them with multiflash setup and reflectors.
I think for getting a minimum chance of success on APSC, you'd need a f/2.8 zoom that go up to 70mm like a 24-70 f/2.8... You'd couple that with a flash that you learn to use with a diffuser like a lumiquest soft box, better if deported. You'll also want to ensure your recognize the venue etc and see how to make the best shoots.
That being said, I managed to get some decent shoots at a wedding with primes only (DA21, DA35 f/2.8, FA77, F135) a metz 44 with a cable to handle the flash in on hand and the camera in another hand and finally a diffuser. But REALLY the wide apperture of the FA77 was of critical use for indoor shoots:
FA77, f/1.8, no flash... And honestly the place (mayor house) had great lighting from big windows.
I'd be really affraid of the 18-135 because for me, while okish the apperture will really not help for both bokeh and low light... You' have to really rely on mastering the flash to craft your pictures...