I think the best way to approach this is not to think about the stat numbers. You think about the game play numbers first.
“How many attacks need to be made to kill this dude”
“How many attacks should be misses”
etc.
Generally rpgs tend to use 3 hits to kill a dude for example. Games handle misses in all kinds of ways. But in general, those 3 hits to kill ‘feels’ good. A really strong guy might take 4.5 hits to kill him, a really weak guy might take 1.5 hits.
This gives you a basic framework to work with and reason about hitpoints, damage and hit chance, etc.
“How many fights does my game have?”
“How many different level ups do I have?”
Divide the two, now you have an idea of how many fights it takes to level up.
With items, you generally want to divide them into tiers.
Tier 1: mega crap
Tier 1a: super crap
Tier 1b: bad
Tier 2: kinda bad
Tier 2a: average
Tier 2b: kinda good…
Same kind of process, figure out how many tiers you have, divide by number of fights in game, etc.
Match all the numbers against some curve, pick a numeric scale that fits whatever you want. Stick x number of zeros in or whatever. The actual numbers really don’t matter, you want to think about the real gameplay stuff above, the actual xp/hp numbers are just whatever feels right.
This won’t give you perfect numbers or a finished product, but it will give you a surprisingly easy to use for setting stuff up and thinking about stuff. If the game is fairly simple, you really might not need anything more at all!