If the system is currently full of fluid, you can simply flush through without a pressure bleeder. I managed just fine with mine. just use two people - one pumping the pedal and the other on the spanner at the bleed nipple.
Do the MS first...
Then calliper furthest from the MS, working back.
If it's full, 1 ltr will do you...20 pumps all the way to the floor, through each calliper should be enough, but you'll see the colour change of fresh fluid.
No harm in getting 2 lts though and doing the sequence twice.
Dot 4 will be fine.
But Dot 5.1 is an upgrade with a higher boiling point...but I seem to remember is is more hygroscopic so will need changing more often than Dot 4 can be left for (should still be fine for every 2 years though)
I'd use Dot 5.1 as it's compatible with the current Dot 4.
Don't use Dot 5.0 though...it's not compatible with Dot 4.