About your questions:
# are bots able to host on garena?
I didn't see any, if i find i'll let u know.
# do we need to set up the bots ourselves or are there servers that have a service for hosting the bots?
Here is the dedicated Ghost Hoster http://cryptwizard.net/
They got servers in at Chicago, IL, USA; Fremont, CA, USA; Secaucus, NJ, USA; and England United Kingdom Great Britian (GB/UK), Europe (EU), they cost 5$ per month or at 25% discount if paid in year advance.
# do we need to get a custom software for the bot? are there any additional payments needed?
I advise getting a lecture at this site: http://forum.codelain.com/
U get info about Ghost++ hosting bot, its free of charge, and also div league bots. Best ones so far.
Also a beginner guide for Ghost++