Which is the range of thrust (minimum and maximum) that a nuclear thermal rocket is estimated to produce?
Knowing these values, which thrust to weight ratio would be able to produce?
The American NERVA program developed a design expected to produce up to 333kN (75klbf) of thrust in a 6.8 ton package, for a thrust to weight ratio of about 5:1. It was never completed or flown. NERVA 2 was supposed to produce 867kN (195klbf) from 11.9 tons, a thrust to weight ratio of 7.5:1. Those are figures for thrust in vacuum.
The Russian RD-0410 was similar in concept, but smaller, producing 35kN and weighing 2 tons, a 1.8:1 TWR.
Those TWRs are for the engine itself, not including any fuel tankage. Generally these engines weren't considered for first-stage launch from Earth's surface, both because of environmental concerns and because their TWR is fairly low; many such engines would need to be clustered to achieve 1.2 TWR off the pad with any substantial fuel tankage.
I believe nuclear thermal rockets are simpler to throttle than bipropellant chemical rockets. I don't know how far down they can throttle but I would guess they can go to quite low thrust values with good specific impulse across their whole range.