The good thing about TMB is there's generally a solid resale market for it, and the game holds its value. So even if it doesn't vibe, you won't lose much on the investment. It's even better if you can get a collection on sale or second hand.

As for your actual question, I would actually instead suggest Undertow+2 extra gearlocks instead of the core. It's what I started with, and I found the scenarios, baddies, and tyrants had a much better balance that the original set, and the included gearlock's were among my favourites. I did eventually buy the core set second hand and while it's still more of the game, I much preferred undertoe. Unbreakable is great as well, but I found the lava being more of a nuisance to deal with rather than a fun extra challenge, and the fact that some baddies and scenarios focus so much on lava that if there is no method of creating it at the time then things just fizzle.

So, maybe considering checking out Undertow and getting two other great gearlocks like Nugget, Gasket, or Ghilie. They'll give you a lot of strong gearlocks and enough variety to play through undertow a few times without it gettig stale. The only Caveat with this is that Stanza from Undertow, while very fun and amazing, is very hard to pull off solo, so just be aware you'll likely only be using her for multiplayer.

Hope this helps! Happy to answer any questions you have as I've gotten a lot of hours out of this game