I've Motul RBF600 Dot4 in mine.
DOT3 is an older standard with lower boiling point etc -- you can safely use a good DOT 4.
DOT4 is most common - But, not all DOt4 are equal - there's a big difference in boiling point between a good and a cheap one.
Dot 5 is a no no unless specifically specified for your car (not compatible)
Dot 5.1 can be used in a DOT3 or DOT4 system, but it attracts water more than others and hence would need to be flushed more often (about every year).
All brake fluid will attract water and that will lower the boiling point, hence need to do a full flush every 2 years or so.
Depending on a the car of course, but on average will take between 1.5 and 2 Litres to do a full flush.
Some cars have a specific sequence in which you need to flush (e.g. front left, right rear, front right, left rear) to prevent pooling of old liquid etc.