One thing MW:O is not guilty of is being P2W. There's no real pay options in the game that grant any kind of competitive advantage so you're fine with all cbill 'mechs. Heroes are nice to grind cash and some of them are solid 'mechs, but for whatever reason they've been very good about not making them necessary at all.
About all you need to spend money on if you continue playing the game are mech bays, and they are extremely cheap. $10 worth would set you for months or more most likely, arguably indefinitely if you are OK with selling some chassis down the line.
Anyway, you don't need to worry about mech bays until you own 4 mechs, but yeah. Just thought I'd say that MW:O's F2P parts is probably the most well handled aspect!