I would be happy with a 407 on 75g tank so long as you arent planning to seriously overstock.
My metric for picking a filter is 1kg of biomedia per 100 litres (25g). So for your 75g thats 3kg of biomedia. A 407 has 4 baskets, each basket holds about 1kg. Bottom basket for sponge and polishing pads, 3 baskets for 3kg total biomedia. I would be happy with that even without the undergravel filter. The only issue with this would be no room for chemical media should i ever need some, but i would just add in a spare filter temporarily with chemical media if the need arose.
An FX4 is overkill IMO
for that sized tank. If you are talking about goldfish and need that additional capacity and turnover rate then a FX4 might be the way to go though. I would start to consider an FX4 at double your tank size, but even then i would probably opt for 2 x 407s for redundancy.
As to cleaning a filter monthly, i try to clean monthly, and by cleaning i mean rinse out the sponges and rinse down the biomedia in dechlorinated water and clean the impeller cover, impeller shaft, impeller, and inside where these parts sit. The impeller is the moving part and where wear occurs and cleaning these makes those parts last longer.
Do you have to clean monthly? No, but your impeller will wear quicker and need replacing more often and sponges will clog more and reduce flow rates. Then when you do clean them it will have a bigger negative impact on your biological filtration than if you do maintenance more frequently.
But its really up to you. Many people opt for less frequent maintenance than i do and have no issues. Personally spending half hour a month to just rinse out some media and wipe down a few parts isnt a big issue.