Sounds like an Edge 500 would be about right, you can get a wrist mount but it’s better suited to on the bike than the wrist. Alternatively look at which of the Forerunner supports cadence meters if you want something which is more running biased.
I have quite a collection of Garmins, my Forerunner 610 has bike and run modes, but isn’t particularly cheap. Edge is certainly best for on the bike, Forerunner 910XT does everything you could possibly imagine, but you pay for it.
DC Rainmaker is probably the best person for reviewing and comparing all the GPS computers around, this is a link to the Edge 500, but he has a product comparison calculator too which is very useful.
Cheers, Rich