Depending on the size of your boat, it's weight and what speed you are looking for with the kicker, generally 10% of the horsepower of the main motor is sufficient for a kicker.
If you are just going to use it to make trolling speed (maybe 3 mph max) then you could go a little less - if you are looking to run it at "no wake" speed, maybe 7-10 mph then you would need a bit more.
Running a 150 as the main motor, if it's suitable for the boat it's on (not over/under horsed), then generally a 9.9 or 15 horse would be a "typical" kicker, especially if you opted for one of the "high thrust" or "bigfoot" models that have lower gear ratio's such that they push heavier weight (at lower speeds) without overtaxing the motor.
If you run offshore etc or in canals with speed restrictions and you are looking for either a motor that will "get you home" if the main one conks out or will push you through the canal with sufficient control under breezy conditions, then a 20 or even 30 or more horses may be required. It really depends on how "YOU" plan on using it.
As to brands, personally I would stay away from Force. They were an ok motor when they were in production, but now they are an orphan that nobody wants much to do with.
Force was born out of the (bankrupt) chrysler marine group. In it's dying days Bayliner boats bought out the outboard division and started building motors to package on their own boats (one stop, boat/motor/trailer pkg). The first models were "Force by Chrysler", then Bayliner formed "US Marine" and the motors became "Force by US Marine" (still pretty much a 100% chrysler engine). Anyhow, a couple years after US Marine was formed, the Brunswick Corporation bought out Bayliner.
Brunswick already owned Mercury so they gave the US Marine division to them. Merc didn't want to continue the Force line but it solved a couple of problems for them for a short time. They now had truck loads of old chrysler parts plus they had warehouses full of really old Merc parts. So they started slapping together motors and putting a Force sticker on them. As an example, in 1991 they made over 30 different versions of the 90 horse Force. When they ran out of 1970 something cases of carbs, they found a different carb and "made it fit", changed the model number and fired up the production line again (31 different sets of parts = nightmare to try and repair).
Anyhow, long story short - if you are looking for a kicker I would first figure out "how many horses is enough" and then look for an older Merc, OMC (Evinrude or Johnson) or Yamaha and stay away from the older orphans like Force, Suzuki and even Honda (which has crappy parts support for older motors).
As to the water sep filter. At the very least it should be changed every second year. Even if the motor doesn't get run that much you may think it's ok to leave it for a bit but in actuality the less the motor gets used the greater the risk of moisture in the tank = the sooner the filter media in the canister loads up.
They are relatively cheap for a new filter = cheap insurance against getting stuck and needing a tow ($$$) or damaging a cylinder and needing a new motor ($$$$$$).
Force shifters "pull" on the cable to increase throttle while everyone else "pushes" the cable.