Great question Villi and thanks for reading our Cisco Data Center & Cloud blogs.
My response is a little long, sorry, but wanted to get you what you needed. I will have another blog late this week of the being of next week that should be of interest to you as well.

We definitely have VDI solutions for your user count, including both C-Series (UCS rack servers) and the B-Series blades, which are the subject of this blog.
To your specific questions:
• You can absolutely use UCS B-Series for your DHCP, DNS, file/print, SQL, Exchange, VDI and more.
• There are NO restrictions on mixing blade configurations, models or applications within a UCS chassis or domain. Most customers standardize on one or two configurations to maximize their HA and burst capacity requirements. See this earlier blog for more: http://blogs.cisco.com/datacenter/fabric-based-infrastructure-and-cisco-ucs-servers/
• We have pre-validated and supported UCS B-Series architecture bundles for VDI solutions (with various VDI software companies), starting at 300 users and up. Your Cisco UCS certified partner can provide you with more specifics. Ask them about part number UCS-EZ-VDI-ENT-VW to kick off your conversation.
• Smaller VDI deployments on blades are, of course, easily doable. We just put together a 300 solution to help you get started.
• UCS “up times” are outstanding and customers can deploy additional chassis and blades, and customize their I/O to each chassis, by themselves. It is very easy to add capacity at any level. No services engagement required or needed.

There are a number of customers with small deployments that are getting outstanding benefits and performance from UCS, whether they are small deployments or small data centers. The use cases vary quite a bit because UCS is very flexible and delivers great value across the spectrum of size.

Below are a couple of case studies:
• Human Kinetics, (this one may be spot on for you) http://www.cisco.com/en/US/solutions/ns340/ns517/ns224/human_kinetics_cs.pdf
• Pitt Ohio - http://www.cisco.com/en/US/solutions/collateral/ns340/ns517/ns224/Cisco_Pitt_Ohio_CS_090810.pdf
• Lindquist & Vennum, http://www.cisco.com/en/US/solutions/collateral/ns340/ns517/ns224/nimble_storage_cs_lindquist_vennum.pdf
• Walz Group, http://www.cisco.com/en/US/solutions/collateral/ns340/ns517/ns224/case_study_c36_647446.pdf

You can access and sort our case studies for companies specific to your industry here - Data Center Case Studies - http://www.cisco.com/en/US/solutions/ns340/ns517/ns224/dc_case_studies.html.

I hope this answered your question and please let us know if we can be of further assistance.
Tom