cottonwool125 Thank you 
With the levels, the higher the better, because the higher the level is the less stardust you need to spend powering it up. Anything under 20 you don’t really want to evolve even if it’s 100% IV because it would take too much stardust powering it up to be any good.
So you could make it amazing, but it would take hundreds of thousands of stardust to make it so, when it would cost you so much less to evolve a slightly lower percentage but higher level.
For example, take a couple of my Charmanders.
Charmander 1 is 618 CP, 67% IV and Level 29; high level and CP but low IV.
Charmander 2 is 458 CP, 87% IV and Level 20; lower CP and much lower level, but high IV.
If I were to evolve and power up Charmander 1 to Level 30, it would be 2,173 CP and it would take 10,000 stardust.
If I were to evolve Charmander 2 to Level 30, it would be 2,256 CP but it would take 75,000 stardust.
So although the IV is much higher, it would take far too much dust to power up Charmander 2 to be worth it.
TL;DR the higher the level, the less stardust you have to spend powering it up, so you want the highest level possible. Anything over 30 is brilliant.
So while IV percentage is important, you do have to weigh up the level and stardust cost. There’s no point evolving anything under 20 as it’s just too low. Level 20 is the bare minimum and I would always go with or wait for a higher level if you can.
(With your Mudkip, Swampert is a good Pokemon worth powering up, so if you’re not planning on powering it up all or most of the way, I would hold out for a better one. It would take well over 50,000 dust to power up to 30 and there’s not really any point only powering up a Pokemon a bit, as that’s just a waste of dust. If you’re not going to use it or power it up at all then it doesn’t matter though, but I would either commit to fully powering up or not at all; don’t just do a bit.)
Does that help? 
Murinae Thank you
We did not get an ex-raid in our town unfortunately, despite following all the advice and targeting gyms. We’ll try again, but with me not being able to Pokemon I can’t help.
I think climbing over the fence is definitely worth it for the chance at a Mewtwo! 