I think the first thing you need to consider, Bernard, is how you'll be using the lens and under what typical circumstances. If you want the "reach" of a 90mm-ish lens in low light, for example, you'll be nudged toward faster lenses - a 2.8 or 3.5 might not work well for you. Conversely, if most of what you'll be using it for is in bright light, a smaller 2.8 or slower lens might be fine.
I use a 75mm/1.4 for low light stuff, and a small and convenient 90/2.8 with my M6-TTL, for example. Both are Leica lenses, however. The 75mm Summilux has a "signature" all its own.
With a limited budget, though, I'd recommend sticking to Voigtlander lenses, or an older Leica lens, and avoid the Jupiters and such. While I've never used a Jupiter lens, my impression is that they are in the YMMV arena, where one might be OK and the next virtually useless.