The white surfaces are completely smooth.
The top is wood with a thick white lacquer I think.
The sides are plastic with a lip that goes around the rear corner, the sides are all white - also inside.
Both the Beomaster 901/1100 and matching cassettedecks were produced in two versions;
One with aluminum side panels (with veneer) held in place by little grub screws under the top cover
and another with plastic sides (normally black with wood veneer) with the above mentioned
rear lip, held by little taps on the inside(s) that pushes into holes in the radio metal chassis.
This Beomaster is of the latter type and I don't know if a white BM901/1100
was made in the aluminum version.
I only know of this one in white and it's not for sale. 
It wouldn't be too hard to convert a BM1100 to white, I think.
The Beocord's shown in this thread are wonderful but actually wouldn't
match a BM1100 which has black alu parts.
They would match a BM901 better, despite the BM901's lower profile.
Beocord 1100 has black metal parts.
Beocord 1101 aluminum.
A Beocord 900 or 901 would be the perfect choice for a BM901 but finding one - and in white
I think would be next to impossible, if at all produced.
Watch out for that collector bug... 
Martin