Anthony Kourtessis from Signal Transformer, a Bel Group, recently went through some of the things you need to keep in mind when selecting toroidal inductors in a short video with us. First and foremost, he said, you need to establish which product will give the greatest energy storage capability. In 
addition, you’re looking for a unit with the lowest temperature rise and possibly the lowest DCR (dc resistance). Perhaps more basically, most toroids go into a filter application, and they must be spec’ed so they pass the frequencies of interest and reject the others.
Candidates for these jobs include a new series of high-current toroidal inductors with inductances ranging from 10 μH to a 1,000 μH, current capacity from 2.4 A to approximately 20 A, and an operating temperature of -55 to 105°C. Kourtessis says these units have great energy storage capacity due to the distributive gap in the core material. They also sport a low current density thanks to the use of super-heavy duty copper that exhibits low losses.
the application I have has a dc current of 2A and a low frequency ac signal of about 10 microamps (200Hz-1000Hz I can choose the frequency). I need to reject the dc. I was thinking of transformer coupling the signal and then use a lockin amplifier to detect the ac signal.. Can u recommend a suitable transformer. Will like the input impedance to be ~1 ohm or less.