900 users... can easily be handeld by a normal system.
@k3rtop: try soyoustart. They are within your budget and should be enough to handle the 900 users.
as for larger boards:
try thinking about virutalisation of your systems.
heres my set-up for handling 4k users:
1 database-server which runs on ssds and uses mariadb (and not mysql) - (16gb RAM, 8vCPUs, 150GB SDD Raid1, no Logging)
2 nginx-server which runs nginx and hhvm (not php, since this will cost too much ressources) - ( each 4GB RAM, 30GB SATA HDD, Raid 1, 8vCPUs)
1 storage-server which will deliver the storage to the two nginx-server and will always provide instant synced files for each instances - (4GB RAM, 500GB SATA HDD, Raid1, 2vCPUs)
1 java-server for handling elasticsearch (4GB RAM, 100GB HDD, Raid1, 2vCPU)
1 server for haproxy which will handel incoming requests and splitting them up to the 2 nginx-servers by setting a "location" cookie (2GB RAM, 20GB SATA HDD, Raid1, 2vCPUs)
additionally there are a few frontent-servers spread over the world to reduce latency, which have enabled nginx-caching. (here we got 2 in USA - each on one side, 2 in Asia - Japan and Singapur, as for EU we use direct connection)
though after switching to hhvm the 2 nginx-systems aren't really necessarey anymore, but for historic reasons, i'm too lacy to turn one offline :D
this all works on a ovh-machine
8 CPUs (yes i know, i have added way to much vCPUs)
64GB RAM
2x2TB HDD, Hardware-Raid1
2x200GB SSD, Hardware-Raid1
with around 4k concurrent users, the overall CPU-Usage is 60% and the site is still really fast.
At the lowes time the server servers around 20MBit to our visitors.
So for 900/1000 users, a small soyoustart is okay - if you want to hassle with bringng hhvm to work with xenforo it would probably even run on the VPS-System
And don't forget to activate caching in xenforo
