Advice on BNC cables for beginners electronic lab
Do I need to worry about impedance when connecting the equipment
  together and buy (for example) 50 ohm cables or would straight through
  work OK?


You seem to have the false impression (see my highlight in the above quote) that a 50 ohm cable will incorporate 50 ohm resistors. A 50 ohm cable (coax or otherwise) is telling you about the impedance it will present when using RF signals (greater than 1 MHz) and has nothing to do with real resistance values measured using a multimeter (for example).

All cables have a characteristic impedance (including "straight through" cables) and its impedance is represented by the formula: -

$$Z_0 = \sqrt{\dfrac{L}{C}}$$

Where L is the distributed inductance per metre and C is the distributed capacitance per metre. If you looked at some cables in their data sheets you might see typical values such as: -


L = 0.25 μH per metre
C = 100 pF per metre


Take the square root of the ratio and you get 50 ohms (nothing to do with actual resistive components).