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!