Magneton and Hitmonlee

No opinion on the Magneton - your call if you care a lot about 100%.

For Hitmonlee, I recommend you go try it against a Snorlax. How does dodging feel against a ZH? You'll need to dodge every ZH as it's super effective against you. If you can do it comfortably (e.g. 2 attacks then dodge and repeat with ease) then maybe it's worthwhile. If it's awkward (e.g. only 1 attack then need to pause 0.3 seconds before dodging each time, room for error) then maybe not. So try it out, see how you can do with it. If you enjoy using him on attack, then power him up.

NOPE, don't waste your stardust on junks (even if it's 100% iv). I got a 98% Machamp with best movesets, and it can barely kill a level 30 snorelax (my Machamp is level 39, and I dodge every charge moves). I mean, barely.... so I don't think your hitmonlee can stand any chance lol

Magneton only decent against lapras. Use jolteon for eletric.
In terms of fighting, machamp to me is better with kc and crossc
I habe both lee and champ with perfect moves and high iv. Lee kicks harder but machamp last longer.
If u dont have machamp with perfect moves. Use lee

Rhydon is a much better choice against Snorlax now.
As far as the Magneton and Hitmonlee goes I would try them out vs some pokemon around their level before pumping stardust into them. You can always use the lower CP pokemon to gym build.

Fighting is good damage-wise but not hp-wise against Snorlax, so you're looking at 2 fightings to kill a Snorlax fast. Stick to pokemons with high hp for Snorlax if you want a 1 kill : 1 pokemon ratio.

I tend to use Jolteon over Magneton, but Magneton gets the job done against Lapras and Gyrados if you dodge.

I'd keep it low - Jolteon tends to be a bit too high cp for prestiging whilst Magneton often falls into that nice 1/2 cp spot.

I've had an 80% Magneton with the same moveset since August and I regret powering it up the few powerups I did.

In my opinion, it won't make any difference powering up. It's what I refer to as a special teams attacker. Not for defending (although it's possible I suppose, but there are far better choices no?) I stopped looking for high IV Magnemites to evolve and instead opted for evolving so/so ones for attacking. I have three Magnetons now, with nothing special for IVs, and they don't seem to stand out in their niche roles.

They're weak. Powering up them up full, 100% IVs or not, they'll still be weak. My opinion is to save the dust and just use your Mag(s) as is.

It's a new thing I've been doing since the Ditto/Thanksgiving event. Save dust.

I cut down my powering up by 95%.
Now I have tons of dust and guess what?
My gym game hasn't been affected at all.

You can send crappy Pokemon that you find on the street to attack and still take down gyms.
Why invest dust in Pokemon that won't change your overall game?
Honestly, I've only powered up two defenders since the update: 89% Eggs Con/Psych and 98% SlowBro Con/Pysch.
Even then, I probably could have been fine not powering those two up but I have hopes for them in the future.

Lots of low(er) CP Pokemon are still useful without powering up IMO. Raichi, Jolteon, Scyther, Pinsir, Hitmonlee, etc etc.

tl;dr save your dust and don't really power up anything (yet)

I used to live paycheck to paycheck (in terms of earning dust). Now I have 240k and counting.