I'm starting out in photography and told to buy cannon 7D & lense 70-200 f/2.8. It's for equine photography (outdors, long distance shots etc). I've been looking on ebay and came accros this - Is it similar to the lense I've been advised to get? I realise it's not the same & price is not near but wondering if it would be good enough to start with?
Not nearly. The main problem is that it's a manual-focus lens, which will be very hard for a beginner to come to grips with for action/sport photography. (It's also a slow lens—it has a relatively small maximum aperture—so you would have to use lower shutter speeds or higher ISO settings, and it probably won't work well with automatic exposure. The lens is converted from a Practika mount; it wasn't designed for Canon cameras.) A proficient photographer who had learned the craft in the pre-auto-everything days could probably learn to use it quickly; a beginner will have a long struggle ahead. You have to learn how to focus where the action will be before the horse gets there, and press the shutter button just ahead of time so that the horse will be in focus just as the shutter actually opens.
I wouldn't get too wrapped up in the 7D part. It's a very nice camera and all, but you can easily get by with a 60D or a 650D and apply some of the price difference towards a more appropriate lens. The lens is much more important than the body at this stage; you can always upgrade the body later if your venture results in significant income, but it will be very hard to generate income if you compromise too much on the lens.
The Canon 70-200mm/2.8 is an expensive lens. When shooting sport, you don't really need IS (image stabilisation), so you can look for an older non-IS lens. And don't overlook the also-recommended 70-200mm/4—you might not be able to use a teleconverter, and it won't focus as quickly, but it is a good lens. You can also look at third-party lenses from Sigma and Tamron, but try to stick to the autofocus f/2.8 versions (the f/4-5.6 variable-aperture versions are really not that good for your pictures).
In a word no, that's a film era Sigma zoom that's two stops slower at the long end (i.e. only captures 1/4 of the light the Canon 70-200 f/2.8 would) will be hard to focus and is not good optically (the images would be soft even if they were in focus).
There are some alternatives to the Canon 70-200mm f/2.8, namely the Canon 200mm f/2.8L prime and the Sigma 70-200 f/2.8 OS HSM. Unless you really know what focal length you need I'd recommend a 70-200 zoom, either the Sigma or if you can find a good used Canon non-IS (but beware the lens you're buying could be up to 17 years old and might have experienced a considerable amount of use).
If you're only going to be working in good light then you could also go for the Canon 70-200 f/4 on-IS. I should also echo what Stan says in that you don't need a 7D, you could get by just fine with something like a 40D/50D.
If buying new you should be looking to spend at least £800 on a lens. If buying used it should be around £500, any less than that and I'd be very suspicious.