Firstly, don't buy second-hand track. You will almost certainly regret it especially if it has been nailed down at some point. Also, you will be hard pushed to find anything but old steel track second hand which is nowhere near as good as the nickel-silver track which is now standard. As others have posted, Peco are probably the best bet, having by far the biggest range of trackwork at various levels of fineness.
As for locos, you may find that the types you actually want are only available from one manufacturer - there is little duplication of prototypes between the brands. Here's a summary of the brands from *my perspective*. Others may disagree....
Hornby Recent releases have been streets ahead of anything produced 2 or more years ago. For post-steam modellers like me, that meens the class 50. For steam fans, there is more to choose from of the more recent releases. Many of the older steam models are tender-drive which seems to be universally hated!
Bachmann Bachmann produce some of the best ready-to-run rolling stock. Carriages and wagons are streets ahead of the Hornby stuff in the main. As for locomotives, all the current Bachmann range of diesels and electrics have central can motors and flywheels and reliable smooth runners. Some of the range has been critisised for lack of accuracy, but for the less demanding like me, they are more than acceptable and good value. I'm less familliar with the Bachmann steam range, but I understand that many of them are excellent.
Lima Lima went out of business a couple of years ago after a long period of knocking-out poor running locos with basic levels of details and accuracy. Hornby have just bought the remains of the company, but it is unclear whether the name or any of the old range will resurface in the UK.
Heljan A relatively recent entrant into the market - has produced some of the best diesel models to date.
The rest..... Dapol produce a range of wagons and the odd locomotive, but the general concensus is that they are not great. I have a couple of Dapol wagons which are OK. If you can't find what you want from the mainstreem manufacturers, there may be a kit out there. These range in quality and price and construction methods (brass, white-metal, plastic, resin)
If you are starting from scratch, I'd go DCC right from the outset. Changing later will be much more difficult. The market leader of DCC in the UK seems to be Lenz who also produce DCC systems for Bachmann. ZTC are a UK-based supplier of DCC.
Adrian