I know where you’re coming from. It’s worth the investment in time and $ to get a good modeler though. Now that doesn’t mean take the plunge on modo if you can’t afford it. That means find a good modeler that you can swing, and save up for the next step.
You could work with a free modeler, like Wings 3d or Blender, while saving up for something like modo. That or get a lower cost one like Silo. Now I only have experience with modo, so that’s all I can comment on, but I do hear good things about those others.
You just gotta decide if you need that awesome renderer and toolset or not. If you can get by with a different renderer or by doing render passes in ZBrush (which outputs nice results), then all you really need is a way to model hard surface shapes. That can be cheap, free, or even done in ZBrush sometimes!
The downside to ZBrush rendering is doing things that require true light physics, like glass, shadow casting, volumetrics, subsurface scattering, hair/fur, particles… modo is even lacking in particles (smoke, fire, etc.), fluid simulation, and a true hair system. However, the fur and the way the materials react with the light is brilliant. It’s quite a step up from ZB rendering.
I also suggest considering how “loose” of art you’re wanting to produce. The more photo real, the more you need a good render engine. The more loose and freehanded, the more you can just draw that stuff in. I find myself somewhere between. I like realism, but it has to have that freehand or it’s too perfect for my taste. Remember this gives you a lot of slack on the science behind 3d rendering. Play it to your advantage.
In addition to ZBrush, modo, and Photoshop, I also use Corel Painter 11 often. It’s choppy as hell compared to PS, but has a ton of good tools and uses. I also just added 3d Coat and Groboto to the arsenal. Both are very cool. While they’re also quite handy, the main ones are ZBrush, modo, and Photoshop.
In the long run, I would definitely plan on saving up for a good package such as modo that offers the beauty shot renderer and features as well as good modeling tools. Just grabbing some modeling tools could probably get you by for now though. Art’s more dependent on the artist than the software, ya know?