How old is the bike?
What rear derailleur you use depends on the size of the cassette. My 520 had a 11-32 or 11-34 cassette, I don't remember which. Road rear deraillers typically only handle up to a ~28 large cog on the rear cassette, MTB rear derailleurs can handle up to 34 or more, my 520 had a Shimano LX derailleur on the rear. Shimano LX or XT will be planty durable and not too spendy.
If you need a new crank set I see Nashbar has the Shimano LX trekking crankset on sale for, I think $69 not including the BB. Gearing is 26/36/46, half way between the std 22/32/44 MTB and 30/42/52 road cranks.