TDR Device parameters
Are you just making pcbs and want to measure the impedance at 1Mhz?  If so I think there are some cheaper boxes that will work but I've never used them.

If you're doing high speed engineering work the sparq is awesome.  It's a TDR and a s-param extractor, like a network analyzer but easier to use and built for pcb guys.



I've also used the tek sampling scope with TDR option.



The main thing I'd look for would be edge speed of the test signal, the faster it is the more detail you're going to see.  After that software and what it can extract for you.  Oh and probing both these tools are SMA input and output which means your test card should have SMA connectors on it.

If it doesn't take a look at gigaprobes extremely highspeed probes for use with the above kind of TDRs (or scopes).  I've used them to get s-param and impedance measurements for motherboards from the intel processor connector down the pci-e pins.



I found the Lecroy and giga guys very knowledgable and willing to help, but none of the above is cheap!