A code reader that will check the ABS & seatbelts is about £200. Something like a Launch CRP123
Launch CRP123 Premium
Expensive for a one-off job, but can be used on many different cars. The next model up can reset electronic handbrakes (which we don't have) and the steering angle sensor- which we do.
Be careful with Amazon & eBay, there are a lot of clones of these type of units, Autel & Launch, that are a lot cheaper, but don't work, won't update and the official manufacturer wont assist with.
As regards the spare- up to you. I threw out my spacesaver and squeezed in a full size wheel. At the time I thought it was a good idea in case of a puncture hundreds of miles from home.
Now I quite like the idea of the space saver- it fits in the space, secures down properly, is lighter to pick up, saves fuel etc.
If a new spacesaver tyre is cheap enough I'd likely get one, punctures severe enough to need the wheel swapping are rare, usually a top up with a 12v compressor is enough to get to a garage.
I suspect a new spacesaver tyre isn't cheap though, I've never heard of anyone changing one (even when they are 14 years old like mine)....