Coming from a turtle background, I can tell you that of all the reptiles available to you, they're the most difficult, time consuming and expensive to keep in captivity. You need to have a solid grasp of water chemistry, how it works and how to alter it, (do you have any aquarium keeping experience?). A basic turtle setup is going to cost you $1,000+ to do right, then there's the ongoing running costs. Your electricity bill will increase significantly as will your water usage.
All that aside, turtles kept indoors take up a lot of time, (I can't remember my last holiday), unlike snakes that have minimal UVB requirements and a simple diet, turtles need to be taken outside to bask daily in the sun, even when using artificial UVB lighting. They, (each individual species) have a complex diet that needs to be replicated as closely as possible and if housed and kept properly, the average freshwater turtle lives between 80-100 years.
You'll need to decide whether you want to keep a long-necked or short-necked species, both have very different husbandry/dietary requirements and there's species that are considered suitable for newbies and those that newbies should steer clear of due to being more advanced.
The other thing to remember is there's no such thing as a penny turtle... all Australian Turtles, apart from Western Swamp turtles and Macleays grow upwards of 20cm, many species attaining an adult size of anywhere between 26 and 40cm with several exceeding 40cm.