Great save and you're a good person. Most people would not bother.
Turtles grow at different rates... but from that picture, it is possible it is 12 years old. If you have a picture with a ruler or something to compare it with, I can give you a better estimate. It appears to be female, but if you show a tail, it can be verified.
Actually RES are bulldozers and sadly can take a lot of abuse. It's hard to screw them up even in the poorest conditions. Although the previous set-up sounded bad, but it had one important point: a basking area where it could dry off. If it did not have that, then it would have likely had some fungus on the skin and/or shell.
If the RES was fed pellets, it would explain why the shell is decent. Contrary to popular belief aquatic turts do not NEED UV light and can get viamtin D from the pellets. So, I am not surprised this RES is doing well under the circumstances. What happens more often with rescues is that they have pyramiding (lump shell) caused my overfeeding.
Some turtles are more skittish, but most RES are pretty friendly. I think with time, it will get used to you and its new home. But often captive turtles will dive off the basking area when somebody approaches and push a corner. That is normal and it will get used to humans soon enough that it will be taking food from your hand.
I'd save your money on the 70G. If it is a female, it is not finished growing. If it is a male, it is not finished growing either. If you cann afford it, go for a 100G. Look in various classifieds, pet stores'posting boards and fish groups for used large tanks. They are around, but it takes time to find it. If youhave the room in your basement, you can get a preformed pond. Kiddie pools'plastic are not as good and will likely start wearing after less than one year.
I rescued a male RES that was 7" SCL (straight carapace length and a female that was 10" SCL. These are common sizes for adult RES. 70G would be OK, but will need something bigger down the road.
Skip the 304... too small. If you want Fluval go for their biggest at at 404. But I recommend you get the Filstar XP3, bigger and better filter which WILL be needed. It's a few doallrs more. The cheapest I have priced it is on J & L Aquatics in Vancouver. Do a web search. If you go with a pond route, you'd have to build your own pond filter, since these aquarium canister filters require the water to be higher for the syphoning effect to work efficiently.
Pellets should be the staple, but you need to introduce greens... best greens are aquatics plants or dandelions. Various land lettuces are not as nutritious, but can be feed. You have two options and both involves hunger. Feed it nothing except greens until it accepts it. That is the stubborn way and it can work. Aquatic turtles can go weeks without eating. The other way is to feed the pellets once or twice a week and always leave greens in the tank. I find going out to local ponds and rivers to collect aquatic greens is nice on the wallet; as well as collecting wild dandelions. Just make sure you rince them off clean and get any leeches or snails off of them.
You can feed some live prey, but it is not absolutely necessary. But it's fun to watch I tell you. Do limit the live prey, b/c most of them time, they're not healthy in larger amounts.
Good luck and this is one lucky RES.
Cheers,
JJ