True, Guy. My rule of thumb: if I can afford to upgrade (instead of renting) and it's something I'll likely use again, buy it. More money up front, but it's an investment.
Lens whacking is somewhat shaky by virtue of it's style/technique, however you could try filming at a higher frame rate (i.e. 60 fps) and then conforming it to your target frame rate (24fps?) which would slow the overall motion of the footage resulting in a smoother image.
The blurriness is something which may take time, as you grow accustomed to how your different lenses respond at different focusing distances. (Focal length, aperture, and the distance your focus is set at being the main factors in addition to how far you move your lens from the sensor.)
Value is the key, which is tricky since it is both created and perceived. Above all, start charging now, lest you devalue your work (and the market itself). You'll learn a lot by just starting somewhere.
Here are two ways you might go about finding your estimate.
+ Overhead/fixed costs (insurance, advertising, etc.)
+ Rental fees
+ Contracted labor (sound engineer, editor, etc.)
+ Travel expenses (gas, tolls, meals)
+ Project-specific expenses
+ Your desired salary (how much do you actually want to walk away with?)
Or.. compare your competitors' pricing, pick a number in that range, and figure out the expense breakdown through the process. (A risky, but stretching method that will often teach you the hard way that you need to raise your rates.)
It might be beneficial to talk with a business-minded friend who can help you think through your business model and what expenses can be written off as tax deductions. (I studied business in college and am still learning the finer points of this whole beast.)
Like Guy said, relationships are central to good business (or really any endeavor in life). Trust plays a huge role, and although it's initially somewhat counterintuitive, setting a reasonable price helps to establish that rapport. (Consider: when you see two comparable lenses for $1,000 vs $300, you might stop to wonder why the one is cheaper. Is it really as good?)
Do as much research as you can, and then simply begin! I've learned so much by charging too little (rarely too much) and learning the hard way. Value-based pricing (check it out sometime) is a difficult yet worthwhile mentality to use when determining your rates - but at the end of the day, there are still hard costs which must be covered.