You can use any 60mm/2.5"/65mm diameter spring you want, it doesn't need to have an ohlins sticker on it, probably about 7" long based off memory although a half inch longer or shorter should be fine, probably safer to err shorter on these. Eibach/Hyperco/Swift springs are about $70-$80 each, often cheaper if you can find a motorsports for sale/trade locally.
For spring rates whatever works for you is what works for you, I've found this car to be relatively insensitive to spring changes, you do it when you're hunting for that last bit of grip, given the questions you're asking though I'd just follow the Ohlins manual and leave the 4k up front 3k in rear, that setup probably accounts for some amount of bumpstop use despite it not making a ton of sense in a vacuum.
If it came with all the parts I believe there's a spring adapter for front/rear to mate to stock top hats, if you don't have that you may need to find it, fab it, or go aftermarket top hats designed for 60mm springs, although for a non-dailyed autox/hpde car, get dat camber bro, it's so worth it.
https://uploads.tapatalk-cdn.com/201...cdb009f7c8.jpg
imho good call to set it up near max ride height, the car suspension is not happy when too low.