1) What brands are better - we have received quotes on a Yale, CAT, Nissan, Clark and Mitsubishi.?

almost everyone on here will push the brand they work for, or what they know most about, as this is natural to consider the brands we know about to be "best". "Best" actually has different meanings in different applications. are you considering only brand new?
if you decide on a used electric, I would plan on purchasing a new and very good quality battery, as the battery will be a very large portion of the value of the truck, and the better the battery (and how well it is matched to the truck and charger, and maintained), the happier you will be with what ever brand forklift you purchase.

2) Are any of these types better in getting parts for them..service as well as pricing?

about the same, pretty much

3) Would we need a side shift hydraulic option to use clamps in order to pick up our bales?
I would think you would want a 4 spool hydraulic option, and the better quality side shift option (not all side shifts can handle a clamp's side force). I would also think you would want the "chopper controlled hydraulic motor speed control option"

4) Is there any special wiring that we would need to plug the electric machine up with?
there is a charger that will need considerable AC power, some are 3 phase 480 volts, 30 Amp lines, the charger plugs into the battery on the electric forklift, and has a connector that matches the truck and battery

5) Is a 36V battery powered forklift the same power as our LP forklift?
roughly the same, the LPG may be geared for a higher top speed, and I would recommend you go with 48 volt battery (over 36volt) for top drive speed.

6) Are there good clamps that have a quick dis-connect so we don't have to have mechanics here to switch from clamps to forks?
yes, this is actually pretty common.