Your dice pool is determined by adding the values of your attributes and skills.
The number of dice granted by your attributes are your stat divided by 2 rounded down.
Equipment relevant to a task at hand (such as a weapon during an attack, or lockpicks in a door unlocking) add automatic successes equal to their quality.
Skill dice:
E: 1 dice
D: 2 dice
So, prior to the recent buys:
Str 4 / 2 = 1.5 -> 2 dice
Club skill E = 1 die
Greatclub passive: +1 on success
Club quality 2
Total: 3D + 2 + 1?
Average: 3.2 + 1?
New stats:
Str 5 / 2 = 2 -> 2 dice
Club skill D = 2 dice
Greatclub passive: +2 on success
Club quality 2
Total: 4D + 2 + 2?
Average: 3.6 + 2?
Anyway, in general we can expect to get 3 or 4 successes, with a slight bias towards 4. If that beats the defender's defense, then we get the difference plus 2 in damage.
Defense:
Toughness 4 / 2 = 2 dice (would have been 1 before the buy)
No relevant skills
Armor quality 2
Total before the stat and armor buy: 1D (average 0.4)
Total after the stat and armor buy: 2D+2 (average 2.8)
In order for us to not be totally mangled by the skeletons, their offense had to be like 1D+0. Even at 2D, or 1D+1, it's now impossible for them to do any damage to us.
The skeleton's defense is 1D+1. Yuuka's previous offense would average 2.8 damage per attack against them. Her new offense should average 4.4 per attack. Skeletons probably have 3-4 HP. So at this point we could pretty much wade in and kill skeletons all day, all on our own.
Of course that's not a very exciting way to level up.
An increase in opponent power would either be going to 4 str or D skill. Maybe both, if they don't have equipment. So looking at 3D+1 or 4D offense. That's 2.2 or 1.6 average offense damage. Still lower than our average, but with the possibility of occasionally doing damage. Either one could hit 4 successes, which is the maximum for our defense. If we don't roll the max at the same time, we can take 1-2 damage.
We can survive such opponents getting lucky a couple times, since we still have one potion left, but after that we'd be heading back to the skeletons for basic farming, or ending the day.
Given the option to take two people with us, and that it looks like we're going for that, we're going to want a target that's a bit tougher than skeletons. (If we were going alone, we could do what Ansom did yesterday and just farm skeletons all day.)
A +1 to opponent difficulty would not really warrant a healer. If we go for a +2, though..
3D+2, or 4D+1, gives 3.2 or 2.6 averages, with potentials up to 5 successes. We have a max defense success of 4, which means if they max out, they will always do damage. Further, their average is about the same as our average (2.6 or 3.2 vs 2.8), which means we could expect to take damage about 25% of the time. That's far more than we can keep up with with just a potion, making a healer a must.