The only thing I'm not well knowledgeable on.. Is if I go with 24v system. Do I need the step down converter to use my 5v USB plugs and my 9v and 12v devices. Or will the plugs automatically adjust what I need without needing the converter?
Plugs just pass voltage along, they don't adjust it. With a 24V battery you indeed need a DC buck converter that can put out 12V to avoid frying your devices. One advantage of this is the converter should hold a nice, steady 12V. (EDIT) Many controllers don't regulate the voltage at the load terminals, they just pass battery voltage straight on through. So you'd get something like 14.4V during charging and 11V when SOC is low. Your devices may not like that much variation. I don't know if your Epever regulates the load terminals or not.
Before connecting any devices check the voltage at the Load Terminals. Always double-check polarity, too. I'd connect the devices one at a time. Always connect/disconnect things in the order the manual says (e.g. it might say to always connect battery first and disconnect it last).
One nice thing about the load port is the controller can monitor usage. Some units will report how much your panels generated, how much your loads used, how much went to and from the battery. The controller doesn't know anything about other stuff you connect to the battery directly, though, (e.g. AC charger, inverter, etc.). So it can't tell you about those power flows.
WIth PWM you need to match panel voltage to battery voltage. So with a 24V battery you'd need your panels in 2S2P, with 12V you'd need 1S4P. But MPPT does voltage conversion. Your Epever can probably connect your four panels in series (roughly 72V at max power point) to a 12V battery. It can convert 72V / 5A coming from the panels and send 14.4V / 25A to the battery. Just make sure to obey your controller's voltage limits. For example, the common 40A Epever has a max PV Voc rating of 100V. Your panels are probably rated at ~22 Voc in normal temps, so 4S would be 88V. So far, so good. But Voc increases on cold days. You'd probably be OK down to 0 degree F or so, but you have to do the math. (Or just use 2S2P.)
Details matter when looking to save on your electricity bill. Some utilities have high fixed (or quasi-fixed) fees. Some have time-of-use rates, surcharges and other wrinkles. It's possible to cut consumption 90% and only cut your bill 50%. Commercial customers often pay demand charges, so a 1000 kWh month with one 15 minute period of very high usage might cost more than a 5000 kWh month with no peaks.
Devices like microwaves and compressor motors for refrigerator and air conditioners are notorious for high startup currents that vastly exceed the steady state rating. You have to be careful choosing your devices and sizing your inverter and battery. It's easy to build a system that can't start these devices even though there is enough power to run them once started.
Here is an interesting thread discussing problems some people had starting up a DC fridge compressor via the load ports on a Victron MPPT.
