I live in Bristol too so hopefully I can help.
General advice first is as others have said really:
1) Bike fit is most important. If it's not comfortable you won't ride it.
2) Buy a bike with a good quality frame, fork and groupset (gears). These are the most expensive and difficult parts to upgrade later. If you buy a decent frame to start with you can add better components as and when the desire arrives.
3) The previous years bikes are always cheaper. For some reason manufacturers insist on releasing a new range of bikes every year. Often the only change is a new paint job. In recent years, with prices going up, the replacement model has sometimes even been lower spec'd than the previous years but at a higher price.
4) Try before you buy if possible. As well as testing out the bike fit you'll want to see which manufacturers shifters you like most. Shimano, Campagnolo and SRAM are the big three. I like Campagnolo, but it's literally a Canon/Nikon/Pentax type debate.
5) Factor in extras. With a road bike you'll need a track pump to get the pressure in the tyres. You'll want a helmet. You might want to go the whole hog and get Lycra shorts and a jersey but at the least I'd recommend some padded undershorts for any distance on the bike. You might want to change the saddle. Often bikes are sold without peddles. If you want traditional flat pedals they're cheap as chips, if you'd rather go the clipless route (which i'd recommend) then you're looking at more expensive pedals and some proper cycling shoes to mount the cleats on.
More specific to Bristol:
As you well know the traffic can get heavy, particularly at rush hour. Fortunately a lot of development has gone into the cycle lanes and they're improving year on year. The major danger zones such as the large roundabouts have underpasses for cyclists and you'd be silly not to take advantage of them. If I have to travel at busy times I favour taking slightly longer routes on quiet roads to minimise the risk. In a fight of car v bike there's only ever going to be one winner. Often taking the quieter route works out faster because it negates the stop start anyway.
Away from commuting cycling, there's a few very nice cycle routes completely away from busy roads. I'm fortunate to live with a few hundred meters of the Bristol to Bath railway path, which gives me a 50km route there and back with absolutely no traffic, just dogs off their leads, children running erratically and prams two-abreast across the road
There's another nice route along the Avon out to Portishead. A big part of it is a hard gravel path which is on the limits of what a road bike can handle but in the dry it's no problem at all.
That's about all I can think of off the top of my head but I'll happily answer more questions if you have them!