Buying any bike whether new or used at that price is always going to be fraught with problems.
Don't buy from Argos they're not a bike dealer just a goods shifter. At least with Halfords they know about bikes and you have the reassurance that things can get fixed.
I would rather buy second hand and be prepared to spend a bit on sorting it, which inevitably you'll have to do.
A older non suspension mountain bike would be ideal. A quick search of Gumtree reveals loads in Belfast.
This is an Raleigh Massif from the early 90's, just £30 Antrim Rd. That's the sort of thing you should be looking for, steer clear of full suspension, front suspension is okay if the bike is good order but will be a bit heavy.
Make sure the seat post moves and not seized in the frame.
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I had one of them and loved it , bullet proof.