Brad,
First off, I don't think the zumo 350LM is an older model. In fact, I think it t is actually a newer model than the zumo 6XX models (and less capable, but that is a topic for another discussion)
I would encourage you to instead look for a nice used zumo 550. It is still the best motorcycle specific unit that Garmin has produced to date, IMO. No, the 550 doesn't have stereo bluetooth streaming, if that was a concern, but it does everything else you could want from a GPS and accepts and run routes like nobody's business. I actually own two of these GPSes (one on my bike, one on my son's) after I bought and tried a zumo 660 and was very disappointed.
A 550 with XM antenna just recently sold for $300 on this forum. That is the price point and feature set that I'd be looking at if I were you.
An alternative option would be to go with a nuvi 765T, which is an automotive GPS (read: not waterproof) but has most of the other features of a zumo 660. It would mean playing the "baggie game" for water protection, but the price is typically a bit lower that the 550. The only knocks on that model (in addition to the weather non-resistance) are that the screen is not as bright in daylight, and the bluetooth is A2DP only, which means that any phone that you pair with it will not come through the GPS bluetooth output. But the music and GPS prompts are in A2DP stereo, and it does run routes pretty much the same as a zumo 6XX.