an Fx5 would def be good but i have a 5ft 120 and i have a rena XP3 on it and it works great and is a bit cheaper than the Fx5, i have had the XP3 since i got the tank like 8-10 years ago and haven't had to replace a single piece, i just have it set up a little differently than they suggest, i have taken out the carbon and have the entire middle filled with bio media and then have sponges in the bottom and have sponge in the top part of the top container and bio-media in the bottom part of the top container and it works like a charm.
as for stock that really depends on what type of fish you like more, do you want a more peaceful tank or a more aggressive tank, do you want many smaller fish or just a few larger fish. for more peaceful tank i would do severums and angels with a small school of some type of geo. for more aggressive fish an example would be what i have in my tank right now, carpintis vontehillo, creamsicle midas, vieja argentea, and festae (but these are all juveniles 7in and under, i will most likely have to move around at least one of them once they all mature) i also have a few pictus cats and some senegal bichirs for the bottom but am thinking about getting rid of the sens and getting one delhezi or ornate