the decision to re-prop should be based primarily on whether you boat can get to the proper recommended rpm range at WOT (Wide Open Throttle), not how slow you can troll down to. If you can reach the proper WOT rpm range with your current prop, you DO NOT want to lower pitch because that will make it possible to over rev the engine (and possibly blow it up).
As was suggested above, if you want to troll slower, the cheapest option is a good set of trolling bags (NOT drift socks, they aren't built to withstand the pressures of trolling) I have a set of 28" Amish Outfitter Beefy Bags for my 24' Thompson and with both deployed, they will slow my boat almost to a dead stop at idle (0.3-0.4 mph down speed). You won't need bags that size for your 18' Lund.
The other option would be a kicker.
Tim