Adding linseed oil simply helps smooth the mixture.  You need very little, if you use it at all.  Some people advise against it.  It would not be enough to offer any water resistance when dry.
Whiting (ground chalk) is good.  It adds smoothness and, basically, fills in the gaps between paper fibres.
Sawdust will only make it lumpy.  I would only use it when I want a cheap filler for bulk areas that will have something smoother added on top.  It also slows drying time. 
For the paste I would use CMC (or wallpaper paste if that is not available - see the article on What Paste to Use) and not too much PVA.
If made properly, your paste should be very close to a smooth clay.
DavidO

nobbly nobble nobbles-

do papier- thankyou so much for your advice!!!! i can tell you know what your talking about! ill be sticking close to you if  can from now on until i find my way- you are like someone thats shining a little light at the end of this pot hole ive been trying to crawl through.

sawdust- no wonder my mache has been taken so long to dry- why then does hilary bravo in her you tube bbc video say her mache drys over night when mine takes days with the same recipe- the answer must be in the saw dust type- where the wood cmes from etc- what wood is it- im so glad to hear that i can now dispense with the sawdust- hilary must have put it in her recipe because she liked the texture it made- whilst i prefer smooth texture because im probably a bit conservative.

and the question of linseed oil- sometimes ive been following recipes with it in and not knowing why its going in there and thinking a whole tablespoon seems an awful lot to put in- i cold see that it softened the mache a bit- but i dont think its a good idea to use it to soften mache- using softer paper surely would be better- all it did was make it smell wierd- make it toxic smelling and give it a smily texture- but making no perceptible end difference. its a conspiracy- people publish their recipes with it in so that it can make poor people like me go off buying loads of things from the hardware store- so they can end up sitting in my cupboard not being used.

so whiting i can get from garden centre i believe.

and as for PVA- what are your thoughts on this regarding flour versus pva? ive made this strip bowl with flour glue- its got many layers of old book paper but is now rock solid- you could stand on it- would pva do this? i wonder- also with the flour glue- i put the strip on and it binds and sticks straight away- i cant wiggle or slid it around- its on there and it thickens where as pva seems to make the paper more vulnerable and weak. 

apart from the questio of rats- surely if the mache is covered with glue skin and gesso and acrylic paint- the rats arent going to want to know are they?

can i make a pulp with flour glue? im wondering if this too would make a great pulp- after reading all these different recipes from the web i think it will be funn y f i juyst end up with financial times and flour glue and that alone.

I'm afraid to use flour. We have insects here so bad that the pieces  would likely get eaten before they could dry. I have discovered some new friends in the back yard. Leaf Cutter ants. They're massive! Of course I discovered them after they stripped my daylillies to the ground. So I'm afraid of what they might do to anything smelling remotely like food... I'm glad I have large dogs LOL
I use PVA glue or sometimes wood glue of it something that needs to be really, really hard like furniture or a piece for a float. Wood glue dries especially hard, and sands easily if need be.