The cement will melt the plastic to bond the 2 pieces 2gether, the excess plastic will ooze out and u have to sand it away. Everytime when sanding is involve u'll have to either

a) paint
b) polish
c) both

When u sand/polish, the decals will be destroyed in the process. After u sand there will be scratch marks everywhere and it will have to be covered with paints. Since u dont plan to paint it to begin with and to top it off u already decal it, its better for u not to glue it at all.

Decals should be the 2nd last step. Unless u plan to buy aftermarket decals to replace the current ones, u might as well proceed to top coat it and just finish it as is. No point going back several steps to glue and sand now. Learn from this and plan ahead better next time.

you can check most of the colors here as sprayed in a white spoon. if you got white and planning to do a subtle off-white shades to other parts, I think AS20 will do.

http://otakurevolution.com/content/laymans...aint-references

surface to cover the rough surface after u sand.
base color is to strengthen the sticky of the paint.

if you want white primer, either Mr Base White 1000 or Mr White Surfacer 1000 will do
Mr Surfacer 1000 is gray in colour
but I suggest White Surfacer better as it has more of 'primer' properties

The best place to buy a Tamiya spray paint is of course at Tamiya shop itself. Tamiya Underground at One Utama, a whole range of Tamiya products
One week of event, will go to the warehouse first

as mushy mentioned, you might have more luck finding it in Tamiya Undergroundz
if not in stock, I suggest buying online as online shops/sellers usually have more choices in stock
here is one example http://www.jshobby.cc/goods.php?id=3254

if you want metallic finish usually is black base(gloss type) follow by silver then clear color.can refer to link
but i do like to know if there is another method to do the metallic finishing.

yea thats metallic color.just apply a base gloss coat color before you apply the metallic color to achive better result.

meaning you must apply primary gloss color before apply gun metal.btw i tried tamiya gun metal without any base gloss coat and it doesnt feel metallic at all.maybe you can try this on plastic spoon first.

primary black gloss ->gun metal->racing blue.