Hello! youve had great advice already, heres a bit more!
I would recommend a Madeira cake for one of the teirs, maybe just in vanilla, then do one and add lemon or orange zest, and lemon/orange curd plus buttercream in the middle, then one teir chocolate. If you choose a chocolate recipe that has melted chocolate in it rather than an adapted vicoria sponge, it will stay moist. The good food website that Blossoming recommends has some nice ones. Madeira cakes can be made a week or more ahead. If you find a good recipe, you can take a week decorating a madeira and allow a week for eating and it will be fine. The chocolate cakes that have melted chocolate in also keep for a good week while decorating, and another for eating.
to help cakes stay moist:
dont refridgerate
ice with ready roll icing as soon as you can, preferably the same day as baking to seal in the freshness
find a good recipe and practice it, dont overcook.
Transport the cakes separately so you dont worry all the way to the wedding, stacking properly for that distance takes practice!
If you use a stand that holds each teir separately, you will only have to remove cakes from the box and put on the stand when you get to the venue. Plus you wont have to faff around making the cakes perfectly level before you ice them and getting them exactly the same heights and so on.
Practice covering cakes with icing a few times before the big day, it takes practice to get pleats out, etc. Use a plastic cake smoother.
Polkadots are very quick and easy to do, fresh flowers in a small arrangement on the top look good too. Also check out ebay for hand made wire toppers, they look nice and modern and you can get them in every colour going. A wide ribbon round each cake will look lovely and also cover up any dimples in the icing
you can use royal icing at the back to attatch, and tie a big bow at the front, attatched with RI too. You could even use a brooch on the bow, looks lovely and little work for you.
If you wanted to do cupcakes, you can make them up to two days in advance if you ice them over the whole cake, as others have already said, dont freeze/refridgerate/refreeze, they will probably taste ok, but you will likely have problems with the cases pulling away from the cakes when they defrost which will look messy. I rekon big cakes is the least stressful way ahead 
Good luck!