Not really all — Rares are guaranteed from nearly every pack until you’ve got them all, as I’ve written, but that’s more or less pedantism.
I don’t see why one necessarily should, either. Especially putting aside PvP, I think there’s no reason to not take your time, play it for fun at a comfortable rate — the PvE content is not running away anywhere, apart from occasional time-limited events.
There’s reason to suspect it might be better spent in the future.
By no means. The point was the renown system is a beginning of what should be the endgame for PvE — not the primary way for newer players to jump-start their mercs, at least the way I see the game design.
Besides, some of us have been there — raring to finally max that merc (for PvP in particular, where it matters), only to find a couple thousand excess coins for them a bit later. It might seem urgent, but there’s actually no rush.
You might have seen my point already: the game is excellent from a F2P perspective, although there’s a bit of grindiness with tasks and the like — especially if you’re in no rush, as you say. As for financial investments — ironically, there aren’t many good options for that at the moment, if any.
If anything, managing with limited resources when playing ‘on a budget’ actually adds more challenge and thus more fun. There are some heroic bounties that are hard to do that way, but many of these problems can be solved, for instance, with only rare mercs!
So, if you’re looking for something to play casually for fun at your leisure and you like the gameplay — this’ll probably work for you. I’d advise against investing in packs at all, let alone coins, but if you want some portrait bundle or whatnot — you decide.
Oh, one more thing: some people actually dislike diamond portraits for their propensity to emote and don’t use them even when given for free (from the events, for instance) — just something to be aware of.
Which ones, let me ask?
For more difficult bounties, you’d generally want a party that works well together. If you wish, I could make suggestions how your favourites can be used best and for what kinds of bounties.
Jaina, for instance, fits excellently in a Frost party, which is very strong, too — so you might want to work on her ‘friends’ (well, teammates) if you wanna play with her.
The ‘ideal’ end-game party could look something like this:
Jaina's Frost Party
# Jaina Proudmoore
# - Ice Block Talisman 4
# - Bitter Chill 4
# Balinda Stonehearth
# - Lesser Water Elemental 4
# Varden Dawngrasp
# - Band of Frost 4
# Lokholar the Ice Lord
# - Frigid Winds 4
# Cookie, the Cook
# - Appetizers 4
# To use this deck, copy it to your clipboard and create a new deck in Hearthstone
Some of these can be hard to get (Balinda and her equipment in particular), but not strictly necessary: Jaina, Varden Dawngrasp (you can use the Potion of Ice instead, especially if you don’t have Balinda’s Lesser Water Elemental) and Lokholar can carry you a long way.