I’ve hit the pointy end of my laundry rebuild and now have to solve the issues of building a cabinet for the washing machine and dryer and also installing a Belfast sink, all with standard Kaboodle cabinets. Until I solve these issues, I can’t finalise the measurements for my benchtops.

Issue 1: I’m aiming for a BC plywood cabinet like Hayley from @housetohome has created here, sitting above two sets of Kaboodle 600mm drawers. I have 3 lengths of 2250 x 600 x 30mm marine ply for the uprights; I’ll have to buy some more for the horizontals. I’m not sure how to make the joins with the ply, though. Should I use dowel…or is there another way?
Issue 2: I plan to have my plumber install a Belfast sink into a standard 600mm Kaboodle cabinet, as can be seen here.

The sink will need to be braced but how and with what? Structural pine and is there an optimal way of doing this? I know that I’ll have to cut down the Kaboodle door to fit the cabinet, but I’m not panicking too much about that (yet!).
Any advice on how to address either of these issues would be gratefully received. Thank you!