What turntable in the $2500 range?

In my experience based only on subjective evaluation (no measurements), if your cartridge puts out 1mV or more, and IF the headshell is a good fit for the tonearm, then you will hear no significant difference between a tonearm with a fixed headshell and one with a removable headshell.  Downstream, even from a removable headshell, you can still effect a straight shot to the phono stage with only the headshell contacts in the signal path. But the qualifications inherent in this generalization are many.  (1) Some tonearms are just better than other tonearms, so they will sound better regardless of the headshell, and (2) not all aftermarket headshells are a perfect fit for all tonearms.  You really need to be sure that the fit is correct.  This means when possible use a headshell that was made by the same company that made the tonearm or use top of the line headshells by Ortofon, Yamamoto, Oyaide, etc. Contrary to some of what has been said here, I find that there is an advantage to having a removable headshell in that one is free to select the headshell that sounds best with a particular cartridge. Headshells make as much of a sonic difference as a TT mat, IMO.

And as far as a used TT goes, if Technics is chosen, then I would advise aiming for any of the G series models.  If it is not a G series, then it is either old or one of the lesser TTs in the current lineup.

lewm, I understand your point, but what if the top of the line removable headshell is heavy enough to raise the resonance of the cartridge/tonearm combination to be above say 20 db?  That should cause mis-tracking especially of low frequencies and that should be audible.  I know it is measurable for sure.   

Then that headshell is not a good choice with that cartridge, of course. (I think you mean “Hz”, not db.) Actually, if you play with the equation for resonant frequency, it’s pretty hard to imagine a combo of cartridge and effective mass that ends up with resonance at above 20 Hz. But I suppose it could happen . Also, too heavy headshell lowers resonant frequency.

I do mean Hz, not db.  With my GAE, there are no aftermarket headshells that I have on hand that did not prove too heavy for either the Ortofon 2M Black LVB or the VAS Nova.  I tried several.  Resonance was above 20 Hz in every case.  Ortofon, Yamamoto, and so on.  The OEM headshell weighs 8.0 grams, all the others weigh 12.0 or more.  

Part of my point was that resonant frequency (Fr) is inversely proportional to the square root of the product of effective mass X compliance.  Therefore, an increase in effective mass would lower Fr, not raise it.

I own a couple of Ortofon and Yamamoto headshells that weigh about 10g.  I've never had to shop for anything lighter in weight but those two companies may make some in that category.