Agree with all of the above. Check at the feed store that you have the right comb, skin colour and number of toes on a silkie. It's a bit of pot luck. I've seen mention on here of people getting nice silkies from a feed store/hatchery but also a lot of poor quality birds. If you're able to I'd suggest a breeder where you can see the parent stock is more reliable.
I'm not sure what you meant with your cross- would you be trying to get a bird that looks OEGB but with millfluer colouring (from a d'uccle?)? That would be seriously difficult and take a long time as you have to breed in the milifluer but also breed out the duckwing. There's an old thread on here about the creation of the millifluer pekin (cochin bantam), i think its called mili cochin info, and it took them years and they were starting with all pekins (buff columbian x mottled) rather than having another breed in the mix which would add another level of complexity.
There is no set number of generations it takes to make a pure bird, it depends on how close the two birds are, for example trying to get a pekin (bantam cochin) with silkied feathers would probably take less time than trying to get a barred rock/brahma/phoenix with silkied feathers as they are more similar in body shape etc. It would also depend on how many different genes you're working with (silkied feathers is just one gene, if you wanted silkied, frizzled feathers you're working with two separate gens and you need both to be passed on and if you add more complex colours in then you have a heap more you need to control). Basic theory is the same in all cases, do a heap of matings, keep the chicks most like what you want and cull the others from your breeding program. You can get anywhere if you have enough time/space/money but it might take a lot of all three.