the care of dumbo rats is no different to the care of standard rats.
the mother doesn't treat a kitten differently if their ears are Dumbo, to the rest of the kittens, and you don't need to treat them differently either.
if the bar spacing is narrow enough, ferret cages are excellent as rat cages.
lab blocks and pellets are the same thing. that is, a rat diet where everything is extruded into generic pellets, each the same.
if you're planning on buying your rat food ready made, a quality lab block is the best option, but personally, as rats are omnivorous foragers, I prefer to give them a varied diet, based around a home made Shunamite mix, with daily fresh food consisting of vegetables, some fruit, cooked grains and pulses, and protein from small amounts of meat, fish and eggs.
but a good quality lab block is better than a commercially availiable mix, as they are very poor quality.
maybe 250ml or so. you don't want it small enough to run out, but you don't want it large enough to go stale. although you could use a larger bottle and just not fill it all the way up. water should be fresh daily.
for a beginner I'd say two. although three or more is better, once you are confident enough to introduce new rats. that way, when one dies, the other rats aren't as lonely while you source, quarantine and intro a new rat or two.
although hey, by the end of November I'll have eight. but right now, my male is on his own after losing his cagemate, so this is why three or more is better.
both genders like shoulders. males are more likely to enjoy laps.
both make great pets. if I could only keep one gender I think I'd have to go for males, but then I'd be missing out on my lovely vivacious girls. girls are so active, inquisitive, playful and curious. males are more chilled out and snuggly.
females have almost no scent, and males smell musky, but not unpleasantly so
females are cleaner but also much messier. whearas males aren't quite so meticulous in their habits, but are less likely to rearrange their entire cage or chew things up.
litter tray. yes. rats are easily litter trained.
you can get fabric to cover the bottom of the cage. but in my experience, females like to shred the liners. I find it easier not to use liners or substrate, and to provide a washing up bowl with things like toilet roll tubes, sheets of kitchen roll, and small toys and treats in in order to indulge their digging behaviour.
so long as the base is a plastic tray, it's fine without liners.
parrot toys, and some dog or cat toys, are excellent for rats. things they can carry around, climb on or hide in are great. my females favourite toy is a double rope ring that hangs from the top of their cage. they're always climbing it, wrestling with the tassel at the bottom, perching on it or swinging on it.
chew toys aren't entirely necessary. but they're good to have. a healthy rat will maintain their teeth by bruxing. but they do enjoy chewing . it doesn't have to be toys though. they can have things like chicken bones or dry pasta for example. even cardboard is excellent for their teeth, as, while it doesn't wear them down itself, it encourages chewing, which does erode the teeth.