Tamron lens for weddings - 17-50 VS 28-75?
If you already have a 12-24mm, then it makes more sense (IMHO) to get the 28-75mm. (if finances are tight, as you say) I understand not wanting to switch lenses. I recently shot a wedding (not professionally) using an 18-50mm sigma and I never had to switch the lens on that camera - it was a pretty nice feeling. Therefore, I can see your draw to the 17-50mm lens. It's a good all around focal range. HOWEVER - that was not the main camera i used for the wedding. Most of my pictures were taken using a manual film camera where I DID switch the lenses (I switched between 28mm, 50mm, and 200mm). It was annoying but worth it. I may not be a very good photographer but I take it very seriously (especially at a wedding). If you are going to skimp on photo gear then that means you will need to make up for it by going the extra mile and SWITCH LENSES if needs be. Or, if you are unable or unwilling to switch lenses and/or your lens won't get you close enough to your subject, then you will have to get physically closer. You have to be willing to walk on over and engage your subject (get up in their face - sort of). Some may get self-conscious or find it annoying but you have to be able to work with that. Not sure what your shooting style is but it's something to think about.
Which one should I buy between: a 50 mm and 18-55 mm lens?
50 mm is prime lens and it gives fixed focal length, you can’t zoom in or out. Having a fixed length lenses pushes your creativity further because you do not have to care about choosing a focal length and can focus on other important things also you will need to move around the scene to get the best framing. It is highly recommended for Portrait photography as it gives shallow depth of field. The max aperture on your 18-55mm lens at 50 mm is f/5.6, where as on 50mm max aperture is f/1.8. The later lens allows more light (approximately 10x) than your 18-55mm lens which is ideal for low light conditions. Your images will be slightly sharper than usual with 50mm lens. You can even shoot without flash at indoors during night or you can just use fill flash instead of it being the main light source. Also the price of the lens is cheaper where you will get in around 10k INR. 18–55mm lens is good basic kit lens which is complimentary definitely not a replacement / competition to 50mm lens.