How to choose step up transformers?

Don’t worry about the secondary. That is determined by how much gain you want.

It is only the cartridges impedance you care about which is 18 ohms. This is a higher impedance cartridge. Get a 1:20 or above transformer for higher impedance cartridges. The load impedance concerns the SUT or phono stage. Now, go back to Souter and see if you can pick out the right transformer!

GTE, it is basically a matter of impedance. If you buy a transformer matched to low impedance cartridges you should stay with low impedance cartridges. Same for higher impedance cartridges. I think you will find this very helpful https://www.sowter.co.uk/phono-cartridge-transformers.php.
If you know someone who is experienced with a soldering iron you get the appropriate transformer from Sowter, a british company who makes excellent products, and you have him mount it inside your Luxkit. They are quite small. Then you do not have to worry about interconnects. The transformers are around $100. Even if you do not want to do it that way visit the site as it is very instructive in lay terms.
I have put their transformers in several preamps including my own Conrad Johnson with great success.