Hi,
I think 500ml is much more than enough, even 250ml would be enough on my opinion.
Before disassembling the caliper, just close the relative hose.
Beware that brake fluid is corrosive and the areas that come into contact with it must be cleaned.
A complete bleed of the brake and clutch circuits needs one liter.
Only suggestion is to use, if possible, the same dot (e.g. use dot 4 if your circuit is filled with dot 4, 3 with 3, and 5.1 with 5.1).
Oils of different brands can be mixed without major problems, but being only just a small refill, if possible, I would also suggest to use the same brand.
For having the correct maximum level in the bowl, all the caliper pistons (of the four calipers) should be retracted to their zero travel position.
On mine I have used ate Typ200 brake fluid (dot 4), ate Bremszylinder-Paste (Cylinder Lubricant) on the cylinders, and ate Plastilube (anti squeal grease) on rear of the Brake Pads with good results.
Other brands also make very good products that are not too expensive (e.g. Motul RBF 660, etc. etc.).
But, actually, being only just a small refill, it wouldn't be necessary to buy anything special.
Hope this helps.
Cheers