Sounds and looks GREAT so far!! You can tell they look happier!!
1. The heater, great!!
2. The lights "coming" great!!
3. Basking area...slate is soooo heavy and not all that appealing, but its working for now
4. If you can buy a suction cup basking ramp, it has a nice ramp and nice area for basking, it suctions to the side of the tank. Which is nice because it gives them more swimming space and both lights can be turned toward the side, on the basking area. Then the temp can be lowered a cple degrees.
Lookup turtle basking ramps online. Theyre acrylic with a green cover to make it easy for the turts to climb
5. You can also go to walmart...and get a sticky temperature screen, to stick on the side of the tank. Cheap, small and u can actually keep track of actual water temp at any time.
6. Awesome that you can get "furniture" 4 them!! A couple things are nice so they can perch on them underwater and or hide behind them if they want to.
7, As for substrate....people have varied opionions...from large rocks to sand....but i prefer nothing as do many RES owners. Its wayyy easier to keep clean from food leftovers and poo. x)
8. You will need a water filter, or you will have to do full water changes weekly! The tank will get dirty fast and stink. If you dont have the money for a nice name brand filter ...there are still many nice aquarium filters. Although in the beginning are costly, but worth it. If u go that way...get the largest u can afford . (That way when u have to upgrade the tank, the filter will still be perfect) Too big is better than too small. But...if $$ is a concern...walmart, petco, petsmart etc have decent filters for your size tank at this time.
9. Hang in there!!!! Youre doing great!! It seems overwhelming, but keep reading and youll get it

Soon youll become like a lot of us...."constantly perfecting" our tanks, for fun and purpose

Hope this helps!!! And WELCOME!