Ok, I'm looking for some recommendations here.

Specifically, I'm looking for a handheld GPS that I can use primarily when working out in the field as a Ham radio volunteer at trail runs etc.

Worst case scenario, need to call in an airlift or a SAR team and I need to be able to give them accurate coordinates and/or trail location and possibly help locate a safe landing zone.

Should I get something with topo map capability or will something else suffice?

I've looked at a couple of Garmin's, the Legend HCX, the Vista, etc.

They all seem to be very similiar in features, I think I need something with a relatively easy to use additional memory card feature so I can load a topo map.

I can't seem to find anything with the topo feature already installed, is that accurate or have just not searched enough?

I don't mind spending a bit of money but I don't have a limitless budget.

Any thoughts?