Bearing in mind that 1" box section roll cages will be illegal in all saloon classes from the end of this year I think you would need to carefully consider the costs should one of these come up. A figure of possibly somewhere between £400 and £800 would be the norm to change the cage to tube, bearing in mind all the extra bars in a class 5.
Most cars are now running either Vauxhall or Peugeot/Citroen engines with either Weber or bike carbs or fuel injection. Some will have a fully mapped, ECU system and some will be on distributor.
Gearboxes can vary from a standard box, to a converted box with straight cut gears, or a state of the art GBZ, Elite or a multi gear sequential, all singing and dancing, shift when you want box!
I'm sure you can see from this there is a vast difference between cars available which is why the prices may vary from £3500 to £20000!
What ever you choose I'm sure you will have fun as class 5 is the best rwd class going! Biased? Yes, I am!