I agree that high energy music normally works very well - and if you are unsure of the music that motivates your clients, top 40 and popular radio mixes work well. or, even better - to find longer version club mixes of popular songs on itunes or rhapsody. I t is better not to play strictly techno or house if you are unsure of your clients and you can get multiple 'sounds going with familiar songs.
I also do not think you have to worry about BPM. that is the beauty of teaching a strength and conditioning class. this is not a dance class or a step class. you do not have to be on a beat. also, not everyone will be going at the same pace (as they must in a choreographed class). You can go half time or double time - syncopation. You will be tempted to do faster cardio intervals during a very fast song - so go for it. From my experience it works well to mix all the songs and bpms. the workout changes - sometimes slow, sometimes fast, sometimes moderate - but always challenging and exciting - but not mundane or high pressure to stay on a particular beat.
In My opinion, the instructor also has to 'love' his song selection (or at least appear to love it) - if you are not buying ti, neither will they. passion and enthusiasm are contagious.