Rear Derailleur replacement

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Hi all!
Glad to have found this forum!

ok, I love bikes but I am not so technical- if it breaks I try and fix it, but as far as specifications go I have little to no idea. :confused::(

So, on my Carerra mountain bike the rear derailleur is busted- bent and distorted and not salvage-able.
It is:
Shimano ACERA X RD-M291
(I don`t know if it`s a long or short cage? Is there a way to tell?)

Now, I have been looking online for a replacement part for but have so far not found one.
The best I have come up with is a part on Amazon /uk which is a "Shimano SORA" GS
It looks very similar, would it fit?
What shoudl I do?
I want it working again as soon as possible, it`s just wrong having a bike sitting in pieces :(
Also, the rear wheel has been bent and ended up turning like an "S", I don`t have much funds to replace these parts though so if there is advice please bare that in mind.

Ok, thankyou for your time. :)

For the current Acera RD, look here: http://www.shimano.com.au/catalog/cycle/products/group.jsp?FOLDER%3C%3Efolder_id=2534374302037261&ASSORTMENT%3C%3East_id=1408474395181400&bmUID=1179532467138

Available any bike shop. ;)