I am a macro-holic. ;D
For increasing magnification on short focal length lenses, use extension rings, and cheap ones are fine - Kenko air has the same optical properties as Canon air.
For increasing magnification on long focal length lenses, use teleconverters.
For cheaper and more compact option at normal and telephoto lens focal lengths, a compound diopter screw-on lens is good. I have one I use for a fixed-lens compact camera.
Choice of macro lens depends on your choice of subject.
You will be unhappy if you do product photography indoors and get a 180mm macro lens. Your subject may be outside reaching distance. People who do product photography in the 1/4 to 1/2 x range often find a 50 to 100 mm lens the ticket. An old Canon 50 f/2.5 1:2 macro lens could be the ticket for a few full frame camera-using photographers. (Actually, the Zeiss 50mm f/2 would be the REAL ticket).
For most people, 90 to 100 mm FL is the best macro focal length for full frame, and 60mm is a good length for APS-C. Insect hunters may prefer 150 to 180 mm, or large insect hunters (butterflies and dragonflies) may even use the old 300mm f/4 L +/- 1.4x teleconverter or the new 100-400 f/variable L IS II.
I have the Canon 180mm f/3.5L, it is a heavy beast and needs tripod, monopod, or decent light + high ISO + excellent hand-held technique. Sometimes I use it with the 1.4 x teleconverter - great for shooting snakes, especially poisonous snakes. The thing is, I don't use this lens for much other than macro. If you want general purpose lens, go for the Canon 100 f/2.8 L macro, or if you are using APS-C and if you want to save money and get a good APS-C portrait lens, the Canon EF-S 60 mm f/2.8 . All macro lenses, even vintage film lenses adapted to EF mount, are pretty sharp. I have used an old AIS Nikkor 55mm f/3.5 (inherited) or an old Mamiya 60mm f/2.8 preset aperture film-era macro lens (my own lens from days of yore) as a just-in-case lens when I am doing serious gram-counting.