What solder specs are safe to use when soldering wires to an Mk2a heat bed?
Old-style lead/tin solder has a melting range of about 180 to 250 degrees (the transition solid to liquid is spread out). This ought to be fine for the bed (even at the hottest hotspot) but wouldn't be good in the hotend.

Modern electronics lead-free solder varies since there are several different compositions, but is typically a bit higher in melting point (by some 10s of degrees).

Special high melting point solder transitions around 300C, but you would only need to use this in very specific scenarios.