By drive belt I assume you mean the exterior belt (we call it the serpentine belt usually) that runs the viscous heater, AC, alternator, etc. If so and recognizing that you are on the other side of the pond:
1. the belt is Mopar 5072437AC - alternative Gates K061130.
2. there are 2 idler pulleys (actually 3 if you recognize and count the identical one that's on the serpentine belt tensioner - all those pulleys are easy to get to at any time (easier if you do a fan shroud mod as part of belt replacement - see below) so replacement is not a critical routine matter. However, inspect the bearings by looking for leaking grease or bearing slop and if present replace them. Mopar 5066938AA - alternative NAPA 36101 (just lose the big washer) or Precision Parts EP101. I don't know about the pulleys on the engine but the NAPA ones are steel and the bearing can clearly be replaced given the proper size bearing and access to a bearing press.
3. before you get the job done open the bonnet and remove the engine cover, crank the engine up and at idle look down the serpentine belt from the alternator to the serpentine belt tensioner. If the tensioner pulley is spinning essentially solid then the alternator clutch is ok but if that pulley is bouncing a lot (not just tiny bits of movement) the alternator clutch needs replacement (depending on pricing some over here find it best to replace the entire alternator and FYI don't go cheap on that part Litens is best or Mopar). If the tensioner bearing is ok the tensioner is ok UNLESS it is leaning into (touching) the power steering pulley in which case the tensioner assembly is shot and needs replacement.
viewtopic.php?f=98&t=64278&p=709322#p709322 provides a simple fan shroud mod plus a link to the Aussie version. I did the Aussie version but did not go the screw/strap route. Instead I drilled paired holes on each side of the cut (4 pairs across the top, 1 pair on each side along the vertical cut, and 1 pair on each side on the horizontal cut - used nylon UV stable zip ties thru the holes to secure the fan shroud - works great and no worries about screws in plastic coming loose)