Bushwhacker is a must (especially if you have a gold card for him), for all the reasons mentioned above. I don’t know what you should get next, it honestly depends on what you’re looking for. You mentioned tanky mercs, so if you wanna go for that I’d say Thunder. I hate Fragger’s LMG (one bullet does the same damage as a BR16/Stark bullet but the K121 is hella inaccurate), but his grenade is way more useful than the concussion grenade. But, Thunder is seen as underpowered right now so this might be the time to buy him and ride the buff wave. If you have Sawbonez you don’t really need a medic, but if you want a more assault style medic, try out Phoenix. He has great weapons (Hochfir/Crotzni/KEK-10) and you can heal yourself faster than Sawbonez. Speaking of buff waves, it hasn’t been confirmed but the community has been asking for a Phantom rework, so you can preemptively ride that buff wave if you’d like, or wait and unlock him later. I personally think his power is fine, and that if you have the right mindset when you play him (know when to go invis and know how to flank) you can be very effective with him. His C94 loadout with increased melee damage is incredible. If you enjoy sniping/long distance, then go ahead and get Redeye.
(lol I ramble) Honestly, the 5 starter pack mercs are great, so you already have a good merc pool going. In the end, look at the stats/roles of the mercs and buy the ones you think would best fit your playstyle. If you need a second opinion, look at youtube. I know that the channel kAndyREW (one of the older Dirty Bomb youtubers) does “Should I unlock ____” videos for some of the mercs, so you can look there, or if he hasn’t done one on a merc yet just look up gameplay on his channel. He really helped me decide who to get.
Good luck ~
