It sounds to me like you are just going to digitize RF and try to find
signals. I'll put a scope on the output of my Wellbrook and see how
much signal is present, but I suspect I won't see much on a scope. The
manufacturer knows the antenna will be going to a receiver with gain.
I'm very certain you will need additional gain before the ADC. dBM
should be quite meaningless if you are just digitzing RF since there
will be many signals present. [dBm would make sense if one sine wave
was present.] The sum of all RF will have many sine wave, so the notion
of the "crest factor" is relevant.

One thing to note is that while the Wellbrook is specified for response
up to 30MHz, it is picking up signals well beyond that limit. I have no
trouble getting air band navigation beacons with the Wellbrook.
Obviously it isn't optimal at 3x the factory spec, but you will
probably have to significantly antialias the signal before digitizing.
I bet it is just loaded with FM broadcast signals.You would probably
benefit from adding some sort of AGC circuit too.