I'm considering botox for my palmar hyperhidrosis, is there any long term complications I need to be aware of?

No. First axillary hyperhydrosis is the indication, pulmar hyperhydrosis has not been studied as much---but it work, now the problems pain during procedure---how much and how many sites to inject--some hand weakness can happen.
None. Hyperhydrosis is treated by dermatologists and neurologists since it is a disorder of skin and the autonomic nervous system. I've treated patients for axillary, palmar, plantar, and inguinal hyperhydrosis. Plantar and palmar injections are very painful, but icing hands or feet intermittently can significantly reduce pain. No long-term complications but temporary weakness of the grip can occur.
Yes. Repeated injections for palmar hyperhidrosis may cause atrophy and long term weakness of the opponens, short flexors, and abductors of the thumb and little fingers. The interosseous and lumbricals are less likely to be affected, but hand function should be monitored by your physician performing Botox therapy. Single treatment effects are usually self limited and temporary.
Be aware of dosing. Since the Botox is not injected into the muscle, it should not cause any weakness but if too much product is injected it could diffuse into the hand muscles and cause temporary weakness in the grip. As always, find someone who is experienced in performing treatment in this area.