Where are you planning on getting your ham from? If you get it from a pet shop you will be getting a Russian hybrid not a pure winter white or Campbells Russian.
I've only owned Russian hybrids & Chinese so can't compare them to syrians or robos from personal experience but of the dwarf hams they are the easiest to handle & tame, they do make lovely very interactive pets & I think they are a good choice to begin with, syrians owners may say otherwise lol!
Your tank would be just about big enough, about the minimum space I would consider but do check how high it is as you will need to fit in a 20cm wheel on top of some substrate.
Most Russians really enjoy some out of cage time, I really wouldn't recommend using a ball for a dwarf ham so you will need to complement that tank space with a good
playpen or safe play area.
Definitely begin with a single ham, pairs can be very challenging & all hams live very happily alone, having a single ham also means it will be much more inclined to want to interact with you than a pair would be.
Russian hams love a substrate they can dig & burrow in, paper pellets don't really work well for that, I find the most successful ones are a mix of
Fitch &
hemp, if using just one substrate I would use Fitch.
There a few good dwarf specific foods in the UK, the burgess dwarf mix that Penguin mentioned is one of them, it doesn't have the widest range of ingredients & is a bit heavy on the millet but still a good mix & fine for a diabetes prone species, others are available from Zooplus, the
JR farms mix & the
Bunny dwarf hamsterdream.
I would recommend using one of those muesli type mixes rather than any of the pellet type foods as dwarf hams really love the small seeds!
I would avoid fruit & any sweet sugary treats (even natural sugars) they really don't need it, there's a wide range of fresh veg they love that are safe to feed (I can post a list if you need it) & other seeds they can have as treats along with things like mealworms & chicken for extra protein.
You are right about them being a bit clumsy lol! They are quite good at climbing up things but not down so they generally just drop off!
One of the favourites is an egg box with a couple of holes cut out, toilet roll tubes, coconut hideouts, small terracotta flower pots to dig around in, small cardboard boxes, seagrass tunnels are great too, some may be safe with things like hammocks but do be careful to make sure they don't chew them before putting anything like that in the cage.
Every ham is different with it's own character, if you can generalise at all I would say males are a bit more laid back but you really never can tell quite what you have until you've taken the time to get to know a ham & bond with it!