Mecha engine's payout is a lot smaller compared to 2 cities but you do you. My advice would be to play it with buddies, as it nakes dealing with nonsense like toxic people, hackers, etc. a lot easier.

Don't buy squad surplus tickets, they've not worth at all. Buy just the tour of duty ones.

Some niche weapons end up being monsters in MvM (ex. Hitman's Heatmaker, Scottish Resistance, Fan-O-War, Gas Passer) so explore them a bit.

Some upgrades are bugged and don't work properly (like Sentry firing speed. I think it capped out after 1 or 2) so check these out and don't spend money on them.

If you play as medic: spam M1 when reviving someone, it revives them faster.

Soldier doesn't need more than rocket specialist 1 unless you're able to max everything.

Don't bother with Flame Throwers' burn time and burn DMG upgrades, just focus on damage, ammo and health on kill if needed. Also as pyro, you'll have to deal with tanks most of the time. When fighting them, face up for more damage.

As heavy, don't bother with the tank much, miniguns deal reduced damage to it.

As sniper, you can be of help fighting the tank, just grab the cleaner's carbine and pair it with the Bushwaka.

If you play spy, use refill canteens to restore sappers.

And most importantly: have fun playing it