First answer this yes or no question: Does your office have fast reliable Internet access?
To explain, if you have good, reliable Internet access from your office than you can take advantage of big budget IT infrastructures in the form of enterprise class hardware fault tolerance and rent a virtual server. You can rent a virtual server to suit your needs quite cheaply as you are charged for the reources your server will have available to it. The virtual server you are then renting will have it's virtual hard disk(s) stored on a storage area network (SAN,) this is a redundant array of physical disks attached by fibre network to multiple redundant hardware servers in a high speed data location such as a datacentre.
To afford this as an SME is prohibitable without renting. You would need to buy three servers an expensive fibre SAN, to get what you get by renting it, double that. and pay for it all to live in a datacentre. Hardware failure would then be your cost past warranty and upgrades too would be your cost. Or pay ten to thirty pounds a month for all that.
If your office has slow Internet access then collaboration would need to be in house and so buying is the way to go. The resources required for collaboration are not then Internet dependant and available in the office when the Internet is not.