The advantage of buying secondhand is that the bike's previous owners will have taken a big hit on depreciation, so you will get a lighter, better-engineered bike for the same money. Provided it's the right size and a good fit, will make it far more rewarding to ride. The downside is that parts wear out and can be damaged on a used bike, so you need some basic knowledge to ensure you don't buy something that's worn out and will cost a lot to sort out.
A lot of the learning comes from experience, so expect to make some mistakes along the way. As you're new to road cycling you're probably going to be better off buying new from a local bike shop. If you strip out all the marketing hype (pro-cycling podium wins, brand history, technology of their top-of-the-line models, etc) there's not much to choose between the entry level bikes – each manufacturer is trading off component quality, cost and weight to achieve a price point and dealer profit margin. So if one £500 bike has higher spec derailleurs, it's probably got cheaper (and heavier) wheels, or lower quality bearings. Any performance improvements that a more expensive/lighter bike could give, you can probably achieve through your training/lifestyle/diet.
Now is a good time to buy because 2012 models are being discounted to make space for next year's ranges. These are likely to be less fashionable models/manufacturers but at this stage in your cycling career getting the biggest discount within your budget is probably more important than anything else because you'll get the best bike for your money.
On an entry level sale, the salesperson may not have a lot of time to help you get a good fit – in the worst case they may suggest that the wrong size is actually right for you. Remember that you're the customer and ask them to make adjustments to a bike's set up so that you can get a feel for it. It's good to be prepared by getting an idea of your ideal set up by using the
competitivecyclist.com bike fit calculator.
I'd also suggest you join British Cycling and get out on their social rides. You will meet experienced cyclists who will be only too happy to share their knowledge.