Yeah £35's not bad, aslong as the seller can gurantee that it is in good working order. If it is, it won't need overhauling straight away.
You know when they're unhappy....you should be able to accelerate smoothly with your foot progressively pushing the pedal. Then, when you nearly hit the bottom it'll feel like the pedal has clicked, and the car should suddenly become much more frantic. This is the second choke opening, effectively doubling the air/fuel mixture being drawn into the engine (it's not actually doubling, but you get the idea)
If the carb wants overhauling, you'll either push your foot to the floor and it'll just keep doing what it was doing already. Or, it'll go mad when you floor it but won't pull smoothly through the rev band, dropping and missing all over the place.
I believe it's not too hard a job to undertake, and like Mike said if you get a K&N you get a new needle to fit too, to keep the air/fuel mix right.
But if that carb setup is working well, it'll be the best £35 you spend on the car. Just make sure to ask the seller if it comes with the black plastic trunking from the airbox to the carb, as your BX won't fit.
Also remember you're going to need all the gaskets and seals to fit it. And lastly IIRC the BX manifold isn't cooled, so in order to get this fixed you just need to get a couple of T-piece sections and plumb into the heater hoses....no big deal, as everything is easy to reach on a TU in a BX engine bay.