Sumitra, with that many users and as you said not all online at the same time, a normal shared hosting would suffice, I have had ran a similar situation in past with hostmonster.com (shared hosting plan @ $4.99-$6.99 a month) without any troubles. though recommended hosting providers are CrocWeb.com (Canadian) (get there mid level shared hosting) based on the SSD and Litespeed webservers.
else, if you want to try a moodle test site before actually signing up, there are 2 sites I would recommend to run a test on, before putting money in.
they are same sites, though based on Hetzner (German hosting company) servers and on nginex webserver running in background, therefore performance is way above any of the shared plans. They give you possibility to open moodle site for FREE as well they have almost every plug-in/popular themes ready installed for you to test drive.