ABS light and Fuel cap light - 1997 Ranger

I bought a 1997 Ford Ranger from a local who inherited this vehicle from an older lady; the guy said he didn't drive it once, but he tried to run it every day. I'm assuming the truck wasn't properly maintained and serviced as regularly as it should have been.. About a week ago the FUEL CAP light came on and I went to AutoZone to replace it, the light stayed on. I disconnected the battery for about 10 minutes and cycled the key 5 times or so, the light went off but not even 20 minutes later, it came back on. Also the ABS light is on. The truck idles ridiculously high, even when I'm sitting still, and the battery kind of fluctuates between high and low. I tightened the bolt on the battery but I am to the point where I'm thinking I may just need a new one?
I've read a bunch of forums online and it just seems like there could be a number of things wrong with it. Some of them have suggested that the ABS sensor be cleaned and the evap sensor be replaced, I was leaning toward those particular suggestions not just because they're cheaper but taking notice to the lack of care of the vehicle it seems likely that she just needs a little pampering.. Does anyone have any advice or similar experience with their ranger, to give me an idea of what could be wrong?
I've read a bunch of forums online and it just seems like there could be a number of things wrong with it. Some of them have suggested that the ABS sensor be cleaned and the evap sensor be replaced, I was leaning toward those particular suggestions not just because they're cheaper but taking notice to the lack of care of the vehicle it seems likely that she just needs a little pampering.. Does anyone have any advice or similar experience with their ranger, to give me an idea of what could be wrong?
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