Definitely recommend the 20mm 1.7. We use it for 80% of our filming.
Also second your thoughts on buying some Canon FD glass. Check out the 24mm 2.8 and one of the many 50mm options - the 1.8 gives lovely short dof and can be had for around £30 with the 1.4 much more expensive. I've now got 9 FD mount lenses and am quite addicted to buying them :-)