Well, this is all back-of-the-envelope stuff, but here’s some guesses. I have been a bit generous to every score; it is probably not possible to maintain all of these numbers, but not all of them should matter to every character.
Hit Points
Varies massively between approximately
5×HD
5th: 25 HP; 10th: 50 HP; 15th: 150 HP; 20th: 200 HP
d6 class, 12 Con, no bonuses
Don’t go under 12 Con unless you’re planning on becoming Undead or taking Faerie Mysteries Initiate or something.
to
12×HD+2/5×HD²
5th: 70 HP; 10th: 160 HP; 15th: 270 HP; 20th: 400 HP
d12 class, 20 initial Con, +6 enhancement, +5 levels, +5 inherent
Yes, I feel that quadratic is necessary to describe such characters. I did assume that Con bonuses were gained linearly, which isn’t even remotely true but I didn’t want to do a table.
Armor Class
Rarely worthwhile to invest heavily in AC; under optimal settings, +1 chain mail and +1 shields are astonishingly common. Thus around 17 AC is about the minimum, with most characters getting around 20, and heavy armor characters getting around 26 pretty easily.
Not very level-dependent; you get most of this AC in the first few levels when you get your first magic armor.
Thus, my answer is
1st: 18, give or take 2; 5th+: 22, give or take 5 or so
Heavy-but-naïve investment can get around 40 but is not worth it, IMO. Too many things can attack you without regard for your AC. Of course, dedicated optimization can get it quite stratospheric, but that’s explicitly outside the scope of the question.
Saves
These, in particular, are on the high side because I’ve assumed a fairly-early Cloak of Resistance. So take these as kind of the high side of “normal.”
Good Save, Important Ability
6+HD/2+2/5×HD+min(HD/2,5) = 9/10×HD+6+min(HD/2,5)
5th: +12; 10th: +20; 15th: +24; 20th: +29
18 initial ability, +6 enhancement, +5 levels, +5 inherent
The min(HD/2,5) is for your standard Cloak of Resistance. They’re fairly cheap and fairly necessary, so I’m figuring you get the +5 by level 10. Might be overestimating slightly.
Good Save, Unimportant Ability
2+HD/2+min(HD/2,5)
5th: +6; 10th: +12; 15th: +14; 20th: +17
10 initial ability, no bonuses
Poor Save, Important Ability
3+HD/3+2/5×HD+min(HD/2,5)
5th: +8; 10th: +15; 15th: +19; 20th: +22
18 initial ability, +6 enhancement, +5 levels, +5 inherent
Poor Save, Unimportant
HD/3+min(HD/2,5)
5th: +3; 10th: +8; 15th: +10; 20th: +11
10 initial ability, no bonuses
Probably want to shore this up with save-specific bonuses, because you may not hit the values here for 5th and 10th, and the values for 15th and 20th are really poor because you’ve run out of resistance bonuses.
Save DC
For highest-level spell:
10+HD/2+5+2/5×HD = 15+9/10×HD
5th: 19; 10th: 24; 15th: 28; 20th: 33
20 initial ability, +6 enhancement, +5 levels, +5 inherent
This is probably the most accurate number I can offer, at least for full-casters: usually they care about only one ability score, which means their DC is going to get pumped at every opportunity. Plenty won’t start with a 20, but probably will start with an 18 if they have any say in the matter, and I didn’t include any miscellaneous bonuses to the DC (Spell Focus, whatever) so that’s a wash.
Maximum Attack Bonus
Full BAB
5+7/5×HD
5th: +12; 10th: +19; 15th: +26; 20th: +33
Full BAB, 20 initial ability, +6 enhancement, +5 levels, +5 inherent
Melee characters can have a hard time getting 20 Strength, particularly if we’re also using the 20 Constitution from above. Getting all of those bonuses to both abilities is very difficult as well. In reality, either this or HP is likely lower. Or, more likely, each is a bit lower.
Medium BAB and cares about attacking
5+¾×HD+2/5×HD = 5+23/20×HD
5th: +10; 10th: +16; 15th: +22; 20th: +28
¾ BAB, 20 initial ability, +6 enhancement, +5 levels, +5 inherent
Similar problems to full BAB characters, and with lack of full BAB the to-hit ability score becomes perhaps even more important. A lot these characters favor Dexterity really, really hard, too, so it can be easier for them.
Medium BAB and does not care about attacking
You just said you didn’t care.
Poor BAB
It doesn’t matter.
Why are you trying to hit armored AC to begin with? At very-low levels you may have to, but then the difference in BAB hardly matters, and otherwise you should be hitting touch AC or saves. Touch AC barely scales at all (for that matter, humanoid armored AC doesn’t scale much either), so by mid levels you should hit that reliably.