What to spend money on depends a lot on play style, but here are my suggestions, in priority order:
- You need $50,000 to buy Praxis kits, period. Just make sure you have $10k available when you enter a new area; don't expect to have it all immediately.
- General weapon mods, because they will always be useful. You don't really need more than three silencers or laser sights, however; there are only so many weapons to attach them to before your weapon choices start to get redundant.
- EMP grenades, up to three or four, for bots, turrets, and cameras.
- Energy bars, if they are for sale. The jar is very wasteful of inventory space, so I never buy any.
- Special weapon mods, the ones that attach to only one or two specific named weapons. You often don't need to buy them, however; you can find them during normal exploration.
If you don't care about Ghost XP bonuses...
- All the other kinds of grenades.
- Ammunition, only if there is one weapon that you constantly use heavily. The most likely candidates are the Tranquilizer Rifle, Stun Gun, and Rocket Launcher; ammo for any of the three is rare, and there is no way you'll be able to use any of them as primary weapons without purchasing ammo.
The only weapon I ever thought about buying was the crossbow, because there seem to only be a handful in the game.
As a side note, mines and mine templates are very useful at the end of your first visit to Tai Yong Medical.
And if you are thinking that the game does not have enough ways to spend credits, you're right. Just use that as an excuse to be relatively free with them.