All depends on the speed offered, as already said.
100Mbps leased line is enough for 100 users. If delivering 25Mbps to each that's a contention ratio of 25:1, far less than ADSL.
Yes, as the backhaul is small contention could be more noticeable but with usage limits this shouldn't be a problem.
Make sure your get 100Mb on a Gbit fibre so upgrade requires no new hardware.
WIMAX? really? didn't think anyone actually used WIMAX, as in 802.16. More likely to be Ubiquiti airMAX which is a TDMA version of 802.11n.
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