Please let's call them MTB pedals for this thread rather than SPD, because SPD is just one particular system (that is quite different from some other MTB ones).
* Some road systems have more float than some MTB ones, and vice versa. Check the number of degrees for the particular models you're interested in. Note that it varies for LOOK depending on the cleats: red 9 degrees, grey 4.5, black 0.
* Even when the number of degrees of float is quoted as the same for different systems, the float can actually be and feel different depending on the design. Some work with more friction than others; some are self-centering.
* Number of degrees to release isn't necessarily related to float. Float is the zone in the middle with relatively low friction, not necessarily the whole amount of movement possible before unclipping.
* A relatively large amount of force to release (compared to other pedals) shouldn't bother reasonably healthy knees. IMO you should ignore this factor as far as comfort goes unless you have a particular serious problem already, and stop and start a lot. The factor is more relevant to security when sprinting, etc.
* Amount of float required for comfort/health is a matter of personal preference. This ranges from zero to as much as possible.
* The lightest stiff shoes tend to only accept road cleats. I suggest deciding what shoes you want first of all, then consider pedal options afterwards.
