I seem to expend way too much energy evaluating camera equipment!

My current vice is audio, particularly microphones. I am producing a promotional video at the moment, using a PD170, and the standard on-camera mic. For reasons that seem less apparent now, I didn't put much thought into audio in pre-produciton, just the complexities of shooting and coordinating.

Now I realise that this was a fundamental mistake (and the majority of you say "no shit")

Well, I am aware at least that i need to get that mic as close to the subject as possible, to make the most of what is a mediocre microphone. It is, however, a challenge to position the mic out of frame, without the assistance from the interviewee or other crew (both not feasible options all of the time)...this also leads to inconsistencies with audio, i think due to the low quality of the mic

This particular project is a promo, but has doco elements, meaning that voiceover is a critical component of the project, as is "on the fly" audio, where a good (as in ability to pick up speech) camera mounted mic would be of great use. I have a decent budget, so I am looking to improve my situation, by purchasing a better mic: perhaps a Sennheiser ME64 (on the advice of Alan Barker); or a lavalier (again maybe Sennheisier, or Sony)... unfortunately at this time, the budget can only afford one or the other.

What would be YOUR advice, o cyber audio guru ??

IMO as a one-man crew you'll benefit most from buying a lav. If you can get it on the interviewee, you'll get instant close-miking with no extra crew member required, and much better audio than the best shotgun on a camera.

To get the most from any on-camera mic always leave the zoom set at full-wide. This will help force you in to the best distance...

Another device to throw in the car is a boom mic stand (this is music gear, not video). In the U.S., an inexpensive one might be about $40 USD. If you have time to set it up, stick your stock sony mic on it and get it in as close as possible. If you're doing anything close to a head-and-shoulders shot you can get it in pretty close. Usually, you'll want the mic above the subject and pointing down, sometimes below and pointing up.

And a good pair of headphones if you don't already have them, and monitor EVERY SHOOT with them.

Adding emphasis to Seth's comments, there is no way to get that broadcast network TV sound with an on-camera mic at any price except in very limited circumstandces (and moving close enough where you can get good sound from a camera-mounted mic is likely NOT to be the best position for good picture composition). The best solution of a one-man band shoot is a wired lav placed on the talent or, if the shot is similar to a news standup and he's comfortable using one, give the talent a handheld mic. If those are not practical, Seth's suggestion of an inexpensive stage mic stand with boom arm positioned just out of frame to hold the mic slightly in-front of and above the talent, along with a good hypercardioid mic like the Rode NT3, can work well. You wouldn't want to use a shotgun because with their high directivity the talent could go on and off mic as they move their head while speaking,