Hiya and welcome.
The Cupra R ST comes with a number of worthwhile extras. Some exclusive to the model, some exclusive to the model in the UK.
Are all R STs ABT, and what is specific to the ABT (or R)?
So ABT Module, it only came with it if ordered at the time. None are new enough now for you to be able to get it retrofitted. But it is a very conservative upgrade. If you want more power, think Revo for a proper (warranty voiding) remap. THe ABT modules has been reported as only being good for 1 hard lap of a track before reducing to stock power.
Do all Cupra have adjustable dampers?
In the UK all the Cupra
280 and above came with adjustable dampers or DCC I think it was called. Don;t think the early 265 3 doors got them though.
Do only R (ABT) have Brembo brakes?
As for the brembos. there are a number of models that came with them. So before the facelift, you could order a "sub 8" pack that came with them. However, were as Europe renamed this pack as "performance pack" the UK just dropped it as an option. So that meant only the special edition cars in the UK got the Brembos (Cupra R,
Carbon Edition and Cupra R ST).
Am I better off buying a 300 lux and then applying a proper remap?
Personally I'd say NO, get the Cupra R ST if you can afford it, just don't worry about whether it has the ABT module. If it does, when it gets mapped it can be removed.
Why do I say this, well a number of reasons.
The extra carbon bits look nice.
The bucket seats (earlier lux's got leather not buckets but this did change to the buckets for later cars.
The nicer wheels.
The Brembos.
BUT the real reason to get the R ST? The standard 4drive compared to the FWD hatch maybe a weapon 0-60, and great in the wet traction wise. But boy it is DULL compared to the FWD with the electronic LSD of the FWD model in the twistys.
I say this going from a FWD
280 ST, to a 4Drive
Carbon edition which doesn't have the suspension of the R ST (unless you mod it

)
Why is the R ST not dull? It has a more aggressive geometry setup all round (-2.0 degree front and rear), and a wider track (10mm front and rear, and it genuinely makes a big difference to the behaviour of the car. This can't be copied without modifying the front suspension.
Get some decent tyres on it and its 99% as good as the FWD hatch in the twistys with all the benefits of the 4drive in the wet and pulling away.
So yeah 100% get the R ST and map it later...