I don't know the dimensions of the Wick you've been using, but I suggest you go about things as follows:
If you're happy with the rim of the Wick, go for a PHC rim that's close to it. You will have a choice of materials for the rim: silver plate, gold plate and white acetal. If you look around for second-hand rims, you may find some in raw nickel and some in black acetate. The acetate ones feel slightly more grippy than the metal ones and of course always feel warmer to touch. Some people have sensitivities to nickel.
You really shouldn't experiment with rims at the same time as cups - you'll never learn anything that way. Get a rim that feels comfortable for you and stick to it while you sort the cup out. Maybe some way down the line you may want to experiment with rims, but that comes later.
As for cups, 21, 22 and 23 represent the middle of the range, the 21 being on the shallow side. The "A" variant has a slightly smaller bore, which gives more resistance; with an Alex 103 you probably won't want that, though you might want to check the bore size of the Wick; if it gives you enough resistance, go for the PHC that's closest. (See the technical specs at
http://www.horncups.com/range.htm)
Finally, you will need a cup with an "E" taper for an Alex 103. In practice, the standard taper will work well enough for you to experiment with, but you'll probably want to get the E taper eventually to ensure the mouthpiece fits the leadpipe properly.
If you're in the UK (or even if not!) you may like to know that Halstead Music has a small range of second-hand PHC rims (
http://www.halsteadmusic.co.uk/shop/accessories) and E-bay occasionally throws up PHC cups and rims.
Finally, this is all a very personal process - finding what suits you as a player. If you have a teacher or valued and experienced colleague these people could help you, provided they don't bring their prejudices with them.