A good inexpensive lens is the Nikkor 300 f/9 - it covers 7x17 and 8x20. Also the APO Germinars are great lenses - 450mm, 600mm, 750mm and 1000mm. The first two will cover 12x20 and the second 2 will cover up to 20x24. Also the 355mm f/9 G-Claron will cover 12x20 and so will the newer 210 Super Symmar XL (very expensive). Hope this helps.
Jumping into this thread because I've been needing good info too . Thanks Fred.