ZCI barrels are superb for the price.
Far better than the aluminium coated barrels like the madbulls. Not as good as the likes of PDI. For the money, the ZCI is an easy recommendation.
There is also the lambda 03. It's a little more than the ZCI. It does, however, come lapped internally.
https://www.ak2m4.co.uk/internal-parts/inner-barrels
ML buckings are good.
I'm having a Prometheus purple phase ATM. Also a very good bucking. It's cheaper as it comes with a standard nub. It's also closer to stock dimensions the ML buckings can sometimes have problems with smaller inner barrel windows.
https://www.ak2m4.co.uk/internal-parts/hop-up-buckings-nubs
As for where to buy. You are right looking at
@ak2m4 His shop sells no-nonsense upgrades, Without the markup. Most of my guns have parts in from him.
Be careful with hop unit upgrades. We had a gun just last week that flat out refused to shoot. Umarex/VFc 416 ,13:1 gears, HT motor, Prowin hop. We tried 3 different air nozzles on the gearbox and it would refuse to feed on everyone. We dropped back in the stock polycarb hop unit and it shot perfectly. Same bucking and barrel. Absolutely no reason for that hop unit to be causing problems, It's visually perfect.