Farmed all as F2P, 37 at 30, and few maxed - 6, though I have a bunch of coins for a lot that I don't use. Won't be enough to max, but for PvE most mercs are pretty useless. Probably get murk-eye maxed when 30, and Rathorian when I get task 7 done. Also close on Tamsin - but I don't use most mercs outside of tasks/powerlevelling them (usually done while farming Valeera coins, since it's an easy bounty to do with 5, and she's nowhere near maxed).
Mildly annoying how crucial Diablo is for a handful of bounties, but other than that, most HC bounties are easily doable with just the same few mercs.