welcome aboard, and well done on what look like a fab Sprint!
you're in the right hands, this is the best forum and lounge bar none for the boxer engined Alfa's, as i learnt many years ago now, and you've already asked such good questions, we'll soon have you suuped up and roaring away!!
To begin with, if you go with K & N's, or any other similar filter set up, still try to duct as much cold air around them too from the grill/bumper areas, for a road car, they do still tend to pick up warm air from the engine bay at slower speeds or town driving, or better still, there have been some great idea's and builds on custom airbox's and ramair intakes too.
as for the braking, as mentioned above, your 89 model will share common parts with the 33, which is no bad thing, because if functioning properly, its not a bad brake set up for the weight of car, but also, much greater choice of upgrades and improvements at much more reasonable prices too, but also as stated previously, the biggest and best brake mods come at the price of also needing to up the size of wheels to accomodate.
a good and common use of parts come from the Fiat Coupe, 16v sized discs, 284mm, for wheels around 15", 20v versions have the 306mm discs if you want to go 16" and bigger, and both have 4 piston brembo calipers which are known to be excellent brakes, plus, the 4 x 98mm stud pattern carries across from the Fiats too.
you will need adaptor brackets for the calipers onto your hubs, but they are not rocket science or re-mortgage to be honest, so no biggie!
as for the radiator, theres not a lot of room left to expand the dimensions of height or width, so the best route is to go thicker, and that means a triple core rad, probably custom made is best.
plenty of places do it, and we don't yet know where you are although it appears from the clues in the pictures definitely in our green and glorious blighty!
theres an excellent place not far from me, ion Walesby, Nottinghamshire, that has produced radiators for our in house racers, namely Sputnik, alfa 33, and very recently for Sprint68, i'm sure you can guess what he races eh!!
welcome again Nev, keep it coming.
all the best,