I don't think FL are any cheaper than FDs, from what I've seen. They're scarcer, and harder to find (maybe).

Just one good lens? Depends on what you want. You said landscapes and street photography, but IMO those are 2 mutually exclusive kinds of lenses.

I'd go for a wide for the landscapes. Get anything in the 20-28mm range. Usually that's 20, 24, or 28, with 28 being the more common (IMO). For street, you often want a bit of distance. If you want to take pictures of unaware people a 100mm maybe. If you want an all-purpose and don't mind being closer to your subject, a fast 50mm will suit you in almost all environments.