Is the basking area 'in the tank' or are you creating a basking area outside of/on top of the tank. If the basking area is in the tank and you have a normal size aquarium, you should be able to find secure screen tops commercially available.
How big are the turtles?
1. Depending on temperatures I would think you wouldn't need a heater outside during the summer in your location.
2. I don't know about those bags to go over the pump, I have heard of at least one turtle getting trapped in one of those bags. I have a DIY filter on one...
Mud and Musk turtles can be pretty nasty to each other, and to other turtles. A map turtle might work, but as mentioned above, musks and muds can be tail biters. If you are keeping two or more musk turtles together or if you decide to add in a map turtle or something similar, I would recommend a...
I use a T5 HO either the zoomed 5% bulb or arcadia 6% UVB for my musk turtles. I also give them a basking area with a flood lamp even though they rarely bask. Musk turtles don't need as much UVB as other aquatic/semi-aquatic turtles.
That is just a reptile basking bulb, no UVB in those. For UVB, it is really recommended to use the linear T5 high output fixtures. I really like the arcadia ones. What kind of turtle/tortoise do you need the lighting for?
Musk turtles need lower levels of UVB than many other aquatic turtles. They also tend not to bask as much. I use a T5 HO with a 5.0 zoomed UVB bulb for my musk turtles.