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I'm going to setup a proper outdoor habitat. I have in mind a 100G rubbermaid stock tank, a Fluval 405 external canister filter, a 200W heater. Going to get some slate and make a basking platform in the middle of the tank, fill the tank 3/4 full. Does this sound reasonable enough?

Outdoor enclosures rock! I wish I had a back yard Rubber maid tank is a good choice but why only 100 gallons ? that's only good if you have 1 male slider. if you are placing only one turtle in there and if it is a female then she would need at least 120 gallons once full grown. Your space would be determined by the number of turtles you have. You will need additional space if you plan to add fish like Koi which you probably should since thats one fish that can survive a RES! plus they are favorites with people who have backyard ponds.

I'm not sure if you should use a regular aquarium heater since they are designed to be used indoors and usually made of glass which again is not a good idea outdoors, you can go with any pond heater there are few different kinds, Im not really experienced with that but I have seen some at homedepot and lowes. Hopefully someone else will chime in on that. Same goes with filters, cannister filters are not designed for outdoor use, they can get damaged if it rains etc..

Go ahead and look up a DIY skippy pond filter, it's pretty easy, cheap and effective! There are lots of DIY that you can attempt since its outdoors you can do trial + error. If it leaks it leaks in the yard so no worries there!

Do you still have a female RES? How big is she now?

I gave my female slider to a neighbor who has a pond, she was really aggressive, I think wild caught. All I have now is Itchy and he's 1.7yrs old, about 2" shell.

Idk If I'd want to put a turtle that small outdoors, but if you do make sure that you have at least a netting over the enclosure.

Which is why I'm only going to fill it 3/4 about 70 gallons. I will of course put a netting or chickenwire on top.

Trust me man that filter isn't going to cut it no matter what. Internal filters are pretty much useless for turtles. it is not a good idea to fill it only 3/4th since there wouldn't be enough water to dilute the waste and you will have to do water changes all the time ( turtles are really messy ). The cheapest option for you here is DIY

I forgot to mention 3/4 of 70 gallons would be only 25 gallons and your turtle at the moment is 2 inches, in 3 months that amount of water would be insufficient. The general rule of thumb is 10 gallons of water per inch of turtle.

The tub is 100 gallons. Going to fill it up 3/4 to about 70 gallons and use a 100 gallon filter. Not the U3, that thing is not good enough right now as it is.

I apologize for some reason I assumed it was a third of the volume. again for turtles you should at least use a filter that can handle twice the quantity of water. What kind of filter is it?