. . . I've never played piano, but I could easily teach myself. I'm fully familiar with music theory. . . .
FWIW (and possibly off-the-wall) --
Definitions that I'll use, to avoid trouble:
. . . An "acoustic piano" is a wood-and-steel piano.
. . . A "digital piano" (= DP) is an electronic instrument with 88 _weighted-action_ keys,
. . . . and at least one sound that imitates an acoustic piano.
. . . A "keyboard" is an electronic instrument with "synth-action" (spring-loaded) keys. They all
. . . have sounds that imitates an acoustic piano (some better, some worse).
(a) I think you're underestimating what it will take, to learn to play piano (or keyboard), with arthritis.
Your knowledge of theory won't help you, much. If your music demands (for example) an E major scale, in the right hand, theory tells you what notes to play:
. . . actually playing them, in time, with controlled dynamics, is a "hard real-time task",
. . . like riding a bicycle.
If you've been playing finger-picked guitar, you'll have some of the fine-muscle control in the RH that's needed for that. But plucking a string is not the same as striking a key, and you'll have to develop new technique to match the new instrument.
So I suspect it's going to be harder than you think.
(b) If you read the threads here, you'll find that "piano sound" is different from "piano touch". A digital piano may have a "good touch" (= "a touch like an acoustic piano"), and could have a "bad tone" (= "a tone not like an acoustic piano"), or vice versa:
. . . the two things are independent.
Similarly, a keyboard feels completely different from a DP or acoustic piano, and most players here wouldn't like it -- but it could have a really "good tone". (Regular readers: Think about a good VST, controlled by a keyboard. Or a keyboard, MIDI-ed into an Avant Grand.)
Since you have no experience playing acoustic pianos, _and_ you have arthritis in your fingers, it's possible that a keyboard, with tone(s) that match an acoustic piano, might be the best thing for you to use. It might be easier on your hands than a DP (the keys are easier to press down), and (since you're starting from scratch) you could probably get acceptable dynamic control of it.
You can get a keyboard with enough keys (61) to play most pop/ folk material, and with several professionally-acceptable piano tones. I'm thinking about a low-end pro keyboard in the Yamaha Motif family -- an MX61, or a MOXF6. (There are 73-key versions of similar keyboards from other makers. I thought Yamaha made them, but they're not in the current catalog. ???).
They're loaded with knobs and buttons and extra tones -- but you don't have to use any of them
. You can just play the first tone that loads, which is an imitation acoustic piano.
(c) Neither the MX61 nor the MOXF6 has built-in loudspeakers. Choosing loudspeakers starts with:
. . . "What's your budget, and
. . . . how much weight can you lift?"
and I don't have any specific suggestions. For practice and learning, headphones or "powered monitors" (e.g. anything from a Roland Cube, on up) are adequate.
The minimum weight for a DP is around 25 pounds, and the P-515 is 48 pounds. The MX61 is about 11 pounds, the MOXF6 is 15.6 pounds.
A P-515 (or one of the other DP's mentioned) is great, but might not be _your_ best bet.