Hey Mark, Unfortunately at that price range there is a little bit of a gap in the market. SD MiniDV cameras are inexpensive but being phased out. Moreover, you hit on a key feature to look for, which is an external mic input. Most built in mics just pale in comparison with even an inexpensive external one.
The real question I think is whether you should try to stretch into an inexpensive HD camera. They tend to start at $600 and you can find used/recertified ones <$500. An HD camera is going to be relevant for a lot longer than an SD camera at this point. And if you have a lot of old MiniDV tapes you want to still access, most DV tape based HD cameras can still play or even record in SD.
So, that being said, let’s look at the options. In terms of brands, Canon, Sony and Panasonic are typically seen as the top 3 brands, and I probably wouldn’t stray from those 3.
If you want to stick with SD Mini DV in that price range or below, the Canon ZR960 seems like a decent camera at $250 with a mic input and a 1/6” sensor. The Panasonics are good too, although they’ve tended to take away their mic inputs. They have however made a habit of putting 3CCD sensors in less expensive camcorders, which is nice. We have a GS500 which was discontinued a couple of years ago which is a nice little camera.
In terms of HD cameras though, we really like the Canon HV20 and HV30. The HV20 was just “upgraded” to the HV30 last year, but you can still find some HV20’s out there for $600 or so. The HV20 is a tape based HD camcorder, and has a mic input. We got an HV20 for a steal right when they were upgrading to the HV30 and we love it. The changes to the HV30 were minimal so if you can find a deal on an HV20, go for it. The HV30 retails for $750 or so, or you can find deals on recertified ones such as this one from NewEgg for $450 - http://www.newegg.com/Product/Product.aspx?Item=N82E16830120360. (If you want to stretch up to a new one, we recommend the guys at BHphotovideo.com http://www.bhphotovideo.com/c/product/539289-REG/Canon_2680B001_VIXIA_HV30_HDV_High.html.)
I’d probably try to go the inexpensive HD camcorder route if I were you.