[DancingDad]

Any cheap OBD reader will read OBD codes and clear codes.
But that is all they will do and codes alone do not give a full picture.
NExt level are OBD readers that will read live data.... that gives a chance of working out why a code is on or why a motor will not start.
An ELM 327 Dongle and one of the free (or very cheap) apps can satisfy this, I am a fan of Torque Pro for android but there are probably others.
Shouldn't be paying more then 20 notes in total

But... OBD readers will only read OBD codes...which are basically emission related codes from the engine ECU.
They will not help with all the other modules/computers in a modern car, things like ABS or brakes
More comprehensive diagnostic kit gets more expensive.
Probably the one I know that seems to cover most functions at best price is the Autel Maxi AP 200 dongle (the white one) Currently on offer from Autel UK at around 50 notes
Drawback is that you only get one free make of car with it.... so your two KIAs would be fine but if something happened with the Mazda, you would need to pay about 20 notes on a years licence for the Mazda add on.

After that, 100 or so notes for entry level kit from the likes of Icarsoft or Launch