How would you balance these classes?

Most of the time, when I see a class like this, the main method of balancing them is through cash flow. At level X you can make stuff from this list for cost Y1-10. You have an overhead or initial cost of Z. The Complete Alchemist for Bard Games was a great example of this.

This is great, and can be rather balanced, but it depends on a certain assumed cash balance, and can be thrown out of kilter when the party says "here's all our cash." Also, if you award money due to some assumption, strongholds, mercenary pay, &c., the output of one character can create a strong drain on that.

So, while level denotes potential and capability for the other classes, it tends to denote only potential for these classes. How do you address that, or do you just not be concerned about it? The magewright / artificer (?) in Eberron had a specific cash reserve granted per level, I think. I remember not quite liking the effect, since they pretty much just made or recharged magic items. I was looking for something with more character, like the alchemist class mentioned above.

I think it's pretty self-balancing in a gold-for-xp game. Even if the other PCs pool their resources then it's taking away from stuff they could spend on strongholds, hirelings, other special gear, etc.

Another way to balance it would be with time limits - if alchemical formulae are like spells, but can be saved for a given priod of time (say, a week or so) before spoiling, then the alchemist can work a lot like a magic-user with very limited spells per day. They can potentially have a lot more stuff available to them, but pay for it with extended downtime.

Personally, I prefer alchemy as an NPC/extended downtime activity, and en the normal economy that handles magic items and other expensive stuff can take care of balancing it.

First, don't assume Alchemists or Enchanters can create stuff by spending gold/silver/whathaveyou.  Make them find the stuff themselves (fine vellum, feathers from a griffon, golden hair of a goddess or angel (or whatever), a piece of wood off a tree struck during a lightning storm, etc.).  That should limit the amount of goods created, and explain why magic shops are less common.

I used to have Complete Alchemist.  I liked the concept, but it worked better as an npc class.

I prefer the Pathfinder Alchemist (more of a bomb thrower or a Jeckyl (sp) and Hyde type).  Not for everyone's game, for sure.

Eberron's Artificer is fairly broken as a class.  Being able to break down any magic item, then use the resulting parts/mana to create a new magic item (for free), being able to cast spells to enchant armor and weapons, and having certain rogue skills (find/disarm, including magic traps if I remember correctly).  I wouldn't use this as a template for anything.

I would make them NPCs because those careers are focused on long hours of research, experimentation and staying put in location. For me, they would be Magic-Users of a certain level who have hung up their adventuring gear to become specialized merchants.

However, Alchemists and Artificers make great patrons for PCs as they need someone else to risk their neck to find the right rare bit of business to build this week's Macguffin.

PC mages are the explorers and battle junkies. NPCs mage merchants have no interest in risking life and limb. There is plenty of money and power in making love potions and charms for the Court.

In my OD&D game, magic items take time. It takes a week per level to scribe a scroll, and costs 100 gp per week of effort in raw materials. Potions can take months to brew (think making wine, beer or spirits, not Kool Aid), and permanent magic items require years of effort. Thus, there is money in being an Alchemist or Artificer, but not for PCs.

In my OD&D campaign, PCs will pay in advance for items they may not see for weeks or months, but that's fine. Travel takes time, adventures take time, and I encourage the "wine, women and song" concept of Gold for XP so kicking back and slow healing is not a bad thing in my games.

ACKS have Dwarven Machinists who:

(1) can only create automata up to (2x level) HD
(2) is paying gp per HD and per special ability
(3) it's a gold-for-xp game, so the two are correlated

In my limited experience it's common for machinists to spend their one starting class proficiency on essentially having a personal obsessive project, which gives them 7kgp of free budget for building an automaton without any laboratory or blueprint. Your wizard-equivalent combatant has a 1HD + 1 special ability or 2HD mechanical attendant at start.

There's a kind of artificer in Majestic Wilderlands.

I have a spellcrafter that can create scrolls, potions and other magic items, in my Basic Fantasy game. He is just starting out but, in addition to spending gp on mundane but expensive ingredients like diamond dust, I plan on requiring him to find rare items like dragon's blood when he starts crafting more powerful items.