If you are employed, wait for an offer before changing clubs. Unemployed you're free to apply to all 'realistic' clubs.
Offers that look blantantly silly are to be rejected. The rule of thumb is 1 league up. That can be stretched a 'little,' but not too far.
My moves in my current game were 2 places, 13 places, and cross leagues to a lower placed team in a higher rep league (France League 1 to Spanish Premier), the two sides were nearly equivalent in the UEFA ratings (actually the side I went to was slightly lower). People start raising eyebrows if your move goes 35+ places.
You can move down as far as the game will let you.