If you choose between 0.5 and 3 m/s you are in a good range.
If it is a diaphragm pump you can suck 2-3 ft head without a problem.

You need to know the fluid, the line size and length, and the desired flowrate to answer this question. These can be used to work out net positive suction head. Fluids may allow up to approximately 10 meters of suction line vertical height depending on their vapour pressure at the working temperature, and pressure losses in the line.

Line velocities (assuming you do not wish to keep solids in suspension, and are not dealing with something like 98% sulphuric acid) of around 1m/s are a reasonable approximate first guess for suitable materials of construction.