Depends on the thickness you are welding.
GENERALLY
0.6mm for datto panel steel which is < 1.0 mm.
0.8 is OK for thicker stuff.
Thinner the material the lower the current and the slower the wire feed.
Use gas for a quality outcome.
READ a lot before starting & PRACTISE lots b4 attacking your datto seriously
http://www.autobodystore.com/new_page_11.shtml
http://www.autobodystore.com/door_rust.shtml

Yes you need to use a .6 tip otherwise the wire will tend to jump around all over the place and make sure that the wire feed wheels are for .6 also or the wire will slip instea of feeding correctly

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Thanks for the links ddiesel. Some good informaion there.

AS I suspected Ill need to get some 0.6 wire and tips, which is fine. They are consumables anyways.

Now the challanging part. Bending the replacement steel to match factory crease lines and bending. I guess thats just paitence and triming, and hammering, and rolling.....

Its a pity im not at home to post a pic of what Ive got left. The vast majority was done for me by an awsome panel beater, but I can afford to pay him to get it all done. (it seems cheapskates are a common theme amongst datto owners......)