Additionally, if you provide these classes, then presumably these classes are going to be used in the game.  You then need to rejigger the game to balance against a group that is some percentage non-combat, with that percentage being anywhere from zero to one hundred.
		
		
	 
Exactly. 
The simplest and best way to handle non-combat oriented PCs is to design a system which acknowledges PCs are adventurers 
first --which in D&D strongly implies, if not flat-out necessitates, combat capability-- and something else 
second. 
Classic AD&D handles this pretty well; after choosing their class --ie, adventuring skill set-- players can roll on the secondary skills table, which determines their normal-person skills, ie, armorer, jeweler, sailor, etc. (while it's a random roll, there's no harm is simply letting the player pick their secondary vocation).  
The assumption is the PC is 
both these things, at the same time, with no ridiculous costing scheme which forces them to, for example, be a 
less capable dungeon-delver in order to be a 
better tailor. 
NPC tinkers and tailors don't need 
levels (NPC soldiers and spies 
do, but that's why you give certain NPCs fighter and thief/assassin levels).
Pay attention to this next bit, it's important! 
Common NPCs need to be 
describable in mechanical terms; things like HP, AC, saves, attack values, skills, etc. They *do not* need a full-fledged chargen system --ie, leveling-- which hardcode 
explicit relationships between those values.  
Picture a master artisan, a glass-maker. HP 6, AC: 11, BaB +0, FORT +2, REF +1, WILL +1, Craft: glass-blowing +15. 
She's described in 3e terms, but without using 3e's advancement procedures. Notice how much more realistic she is? There's no direct relationship between her hit points/combat ability and her ability to create really pretty glass vases? 
Can anyone explain to me how forcing her into 3e's leveling scheme would make her, and, by extension, the campaign she plays a bit part in, better?