I think him just selling stimulants (or tonic; either or) works fine. Something else could also work. I think it's more important that the reward for a resources like those rare coins is static. Unless there are ways, at least theoretically if impractically, to farm those.
Nope, not met the TP dealer. Or rather, I hadn't. Now I have, at the sign at the road through the forest. The descriptions are a bit odd with the %1 notations, which seems like a formatting issue since it just disappears once you buy the items and use them. First impression is that most are still rather weak, but there are some stand-outs. Charger gives little at the start of a battle, but if it goes on long enough you have more or less unlimited. Didn't see any that gives max TP at the start. I also didn't see anywhere where it says what natural TP regen is, since a few of them rely on that.
It might be an idea if classes that rely on TP have some passive skill to increase TP regen, or some other means of getting more. I've not looked through all classes, though, so I don't know if that exists.
Also just noticed that the buff from Heart Palpitations is just 10%, which is barely worth using a turn for, let alone 40 TP. It's better to just attack. I think it lasts four turns, so you get 40% more damage and defence for the cost of 100% damage and 40 TP. For that cost, I'd expect an extra 150%-200% damage over those four turns, so a 35%-50% buff. That said, I think magic for Unkindled is very strong as it is, so the buff isn't needed for that reason as well. But 10% is what I generally classify as "not a number" unless it's permanent.
To make spells more varied, you can add things like partially ignoring magic defence, elemental resistance, or reflection; tweak hit rates (strong that often misses, or weaker that always hits); have semi-multi attacks in various ways, like the main target takes full damage and other enemies take partial damage, a few hits on random enemies, or first hit on target and one or two on random secondary targets; or add chances of causing various afflictions.