I would go with a 90D or 90 equivalent (superfield, digital widefield are volk's 2nd and 3rd gen 90s with some small differences). You can learn about each on their website. I ended up buying a digital widefield for the wider field so you can scan a wider area with the slit lamp without readjusting the lens position. They say it provides better results with photography and imaging solutions but I don't have any practical experience with comparing the dwf vs classic 90D. The DWF also has a shorter working distance which does get annoying when the patient has long lashes; I end up cleaning off the smudges rather often. A side note, NBEO boards part 3 (the clinical skills exam) provides all equipment including video BIO and lenses so you don't need to/can't bring your own lenses in. They have chosen to use the digital wide field for the slit lamp portions so it might help be familiar with it. I'm sure whichever lens you choose, you will be able to do what you need to do.
I would also get a separate higher mag lens like the 60d, super66, digital 1.0 (1st, 2nd, 3rd gen). A 78 would be someplace in between a 60 and a 90 in terms of field and mag. Like some others, I don't see a point in getting a 78 if I already have a wider field 90D(equivalent) lens.
Gonio lens, I just bought both the G3+flange, and G4 no flange lenses recommended by ICO. You might be able to get away with borrowing one but I just bought them anyway because of the hassle. Each lens has its benefits.
I use the 20D and it's fine but it's up to you if you want more field. kind of hard to make sense of it if you've never done the procedure but that's the information :/
My arsenal:
Digital Wide Field for fundus biomicroscopy
20D for BIO
G3 for gonio
G4 for gonio
I've been thinking of selling the 3 mirror g3 sinc ethe 4 mirror g4 will work fine but I'm not sure if I'll ever really find myself in many situation where I'll need to use the retina viewing mirrors.
I'm also looking or something like the 60D/super66/digital 1.0x for increased magnification.
one final note on super/digital lenses, these are progressively more expensive so don't beat yourself up over these, you can get perfectly fine views with any lens in the classic series. Or go buy used online!