I love my 150mm Sigma EX DG HSM Macro f2.8
Second-hand can be had for under £400 - I paid £350 for mine..
I have recently purchase a Sigma 105mm f2.8 Macro EX DG OS HSM and find the build quality to be very good.I'm very happy with it so far. I don't think sigma 's build quality is as nice though? I liked there silky feeling before... Now it just looks plastic.!
So does the tamron 17-50...The build quality is fine, it just feels cheap!
I went straight for the Canon 100mm f2.8L IS when I got a macro lens as I had the money to do so. ......With the IS on it focusses fast enough on both my 5D3 and 70D to be able to use AF nearly all of the time and I have it prefocussed to a subject area for most of the other shots. I have used MF but only on a very few times as the AF speed is excellent and is supposed to be similar for the non L version.
You will need to think about lighting for most macro shots as I usually shoot between f9 and f16 to get a large enough DOF to cover the subject but that will slow down the shutter speed too much (unless you really bump the ISO and then you end up with noise) so good light is essential. You can use a speedlite, but that doesn't tend to give the best light if it's on camera, but off camera flash with a speedlite is better. Alternatively you could get a ringflash or dual flash which attach to the front of the lens and gives a nice even light. My macro shots were greatly improved with a ringflash, I got a Marumi ETTL flash for around £60 to start with but I've now got a Sigma EM140 DG twin flash and that gives a whole lot more lighting options but is a lot more expensive.