I'm going to respectfully dissagree with a couple of the points made by the other Alex, but frankly they are minor points. With regards to my dedication to keeping turtles I might point out that I have been keeping, breeding, rehoming and rehabilitating healthy, happy and highly active turtles of many species for quite a few years now. What I say is from my experience, and from that of many other experienced terrapin keepers. Also, I do not have bare tanks, all my animals are provided with adequate tunnels, wood, cork bark, basking areas and pond weed, I just don't use gravel or expect or expect a fixed planting system.
The gravel issue - gravel can be a problem. Terrapins have been known to swallow it if they can fit it in their mouths, and this can cause impaction which can be fatal. This is certainally not guaranteed to happen, but be aware that it can. Also, sharp gravel can, in some cases, scratch the plastron (underside of the shell), causing damage and infections. Certainally you can have gravel, but it is generally easier not to. If you decide to have gravel then use a round gravel that is big enough that your terrapins are not in danger of swallowing it.
Regarding heaters - again as I have said this is a species dependent issue, however the majority of imported species are from the USA and simply don't need heating indoors in the UK. Common musks in particular have a natural range that extends far north into Canada; heating them is pointless.
I personally wouldn't use plastic plants, but as long as they dont have easily swallowable leaves I guess they should be OK.
Sooner or later sliders will view anything as food, Trachemys are one of the more aggressive and predatory groups.
With Sternotherus species your main problem when it comes to stocking them with fish is water volume. These are species that dont so much swim as walk allong the bottom 90% of the time, and particularly when hatchlings really need quite shallow water to be happy (3 inches is plenty for a baby), so that's really going to cut down your water volume. I use large, shallow containers for raising muds and musks, for example under bed storarge boxes are excellent. On the plus side as long as you have the water volume to support the fish, musks tend to not be able to catch them. Stick with smaller, hardy, fast swimming fish like white clouds or dannios and you'll be ok.
Obviously there are some tank basics you need to make sure you've got - No lid to avoid humidity issues, decent UV bulb, heat lamp, proper basking area that is large enough for ALL your animals to be entirely out of the water at once (though musks don't use them as much as other species) etc.