Most restaurants freeze the fish to kill any potential worms living in their flesh. It shouldn't damage the fish (providing it's not shell fish!)
Sushi rice is expensive. Best bet is to try pudding rice. It has a similar end result. You could also use white wine vinegar to replace mirin. The rest of the products you should buy from a chinese/asian food shop and not a British brand (as the prices are horribly inflated and you only get a small amount) You'll also need a rice cooker to achieve the desired fluffiness of the rice.
I think you should try with crab sticks and smoked salmon/tinned mackerel first - just to get use to the preparation and seasoning, so that you don't waste expensive fish.
Also a word of warning: too much sushi isn't good for you, despite western marketing! The japanese are well aware of the high amount of heavy metals contained in fish, which cannot be processed by your body and often build up and cause health problems. Not to mention the ton of salt that's in sushi!