Got away as low as 30 ppi for a banner, really depends on the content, if the raster image has no detail (water, fog, sky) etc. then you can go quite low, if it's a persons face, building or anything with a lot of detail you couldn't go that low. 150-200 is still quite high for banners, but then again different places different technologies different standards etc.
Anyway - if you're going to draw them as A3 and scan them - then scanning is best done by working to scan to your finished size without digitally manipulating it after the fact.
Width of a pull up banner is about 850mm and the height is about 2300mm
You'd need to scan your A3 which is 297mm to fit the width (as height will be irrelevant)
Scan it at 2.8 x the size you need (2.8x297=c.850mm)
And you need to scan at approximately 150 ppi max
Or put it this way - scan it at 2.8x150 ppi which gives you resolution target of 420
Scan your A3 at 420ppi
Which when scaled to banner size = 150 ppi finished size
If you want more detail or if it's line art scan it at a higher res - but 1200 would be overkill for a banner stand.