I would certainly look at repairing what you have now, Janet.
Don't forget, if you replace the garage, you will have the expense of demolishing and removing the old one as well.
Perhaps take off the old roof and replace with something like this...
http://www.onduline-roofing-sheets.co.uk/
It's available from Wickes, Builders Merchants, and others.
Once the existing has been removed, the top course(s) of brickwork can be easily re-bedded. A piece if timber laid on top of that (called a wall-plate) will help hold them all together. The new sheets will then be nailed to this.
Is the garage for a car, or just general storage?
Can you post a link to any of the alternative garages you're interested in? If not, then perhaps you could give us the Company name and product.