Hi French-guy - welcome to the forum!
ARC (or App Race Control) is essentially a race management system that runs as an app on smart devices - Apple iPhone and iPad, some Android phones and tablets and Amazon Fire tablets.
There are three different powerbases that can use the ARC app:
- ARC One - a basic analogue (one car per lane) powerbase where ARC acts as a lap timer and also enables pit stops to refuel and change tyres.
- ARC Air - a more advanced analogue powerbase with wireless hand controllers where almost all the ARC features are enabled - including a pace car.
- ARC Pro - a digital powerbase (up to six cars on two lanes, plus lane changing) with wireless controllers and all the ARC features are enabled. There is also an 'analogue mode' where the powerbase will act the same as ARC Air and uses the ARC features available with the ARC Air powerbase.
There is a good summary of the three ARC powerbases and the ARC app options available here: https://www.scalextric.com/uk-en/shop/sets/app-race-control-arc.html
To understand how the ARC app works in practice, there is a guide to using the app with ARC Pro here: https://slotracer.online/missing-manual/
I really like both ARC Air and ARC Pro. If you want to race with up to five other people, then ARC Pro is fabulous. However, if it will be just one or two of you racing, ARC Air may be perfect. There are additional costs to digital, most notably a digital chip (decoder) in each of the cars. I'll summarise the costs here...
The C1403 ARC Air World GT set is £199.99 and includes everything you will ever need for ARC Air. You can buy more cars if you want and use your Scalextric Sport track to extend the layout. You may also want to add borders or barriers.
The C1404 ARC Pro 24h Le Mans set is £249.99 and includes the basic ARC Pro digital components - powerbase, two digital cars, two controllers, one digital lane changer and one power supply. It is a really good value starter set, but to use the pit stop features of the ARC app, you will need to add a pit lane (C7014 or C7015). To run more than two cars, you'll be adding more C8438 ARC controllers, a second P9300 power supply, another two or three C7036 digital lane changers (for a 4 x 8 track) and a digital chip (decoder) for each additional car you'll be running. Of course, these extra components can be added gradually and you can economise on chips by swapping the C8515 digital plugs between your DPR (Digital Plug Ready) cars. Otherwise, your Sport track can be used to extend the layout - and you'll probably want to buy some borders and barriers.
I hope that helps answer you questions!