Just to note with the HP power meters: the reason they are so cheap is the number of power meters exceeds the number of mounts/sensors left that haven't been blown up by at least 2 orders of magnitude!
I watched a power meter with three sensors in a nice case go for nearly £1000 here in the UK a few years ago.
Yeah, it's going to involve paying through the nose. But, I understand that the same thermistor mounts and cables also work with the 432 series power meters, so if I end up with one of those too...
Ummm... I know I'm risking having someone try to drive a stake through my scaly tinkerdwagon chest, but seriously... if the thermistor head & cable for that meter is THAT expensive, you should probably cut your losses and move to a civilised machine that uses the 84xx series sensors.
For similar $300-ish cost, they're good 100KHz to 4/8/12GHz or even some models ($600-ish tho) as high as 26GHz. HP 437Bs average ~$100 all the time and routinely show up for as little as $50-60 passing all self-tests; you'll get a helluva lot more for your money there than with that old limited thermistor head. And if you MUST have a analog example, I've seen the 84xx-compatible 435B as cheap as $15 shipped.
Also, once you have your 84xx power sensor, if you need it, the newer E4418A meter are also compatible, and I've seen a couple of those go by for $150-ish, though that was a couple years ago.
Like bd139 says, the big problem is getting a power sensor that isn't effed up; but that appears to be true with your old thermistor head as well.
mnem
*Drops back & punts*