I work for a small IT consultancy, who deal day in and day out with companies of your size - a few more things to consider.
Growth - Putting in a cheaper solution now is fine, however if you grow another 10 users will this cope? Buy a server that has expandability. Ill cover this more shortly, but see it as a 3-5 year investment.
Downtime - If your server is down for a day, and those 10 current people cannot work - how much does that cost your business? Take wages, Project delays, unable to process work, customers losing faith, no emails etc. With this number you are more equipped to determine your actual requirements.
Cloud - You say you have lots of power outages but dont want to go cloud, you should try high impact, low footprint services such as email in the cloud. Outlook in cached mode with sensibly sized mailboxes should be no issue here and any downtime you have on site wont affect the ability to email customers. IMAP mailboxes are OK but very old hat, especially if you want a hot desking setup, POP3 is awful - avoid - o365 if setup correctly end users can do themselves in 3 clicks.
A Business critical server should atleast have the following, otherwise its not that critical to your business:
Redundant power supplies - no explanation needed
Hardware RAID - Avoid stablebit, software raid or anything else, you wont see any gains at this level other than cost reduction, hardware raid is not expensive and plenty more reliable, ensure it has battery backup (usually optional extra but will save your bacon) and a flash based cache.
Enough Capacity for growth - Memory and Disks are cheap, go with more than you need, backups should be sized at 2.5x the data to be backed up as a rough sizing guide allowing for plenty of history.
Virtualise - Hyper-V is painless and you will thank me, it doesn't overcomplicate much and allows you to be completely hardware agnostic if you have a total server failure. Restore those virtual machines onto anything that runs Hyper-V and away you go. Dont install anything on the host OS other than the Hyper-V role and it will be very stable. Windows licensing grants you 2 virtual machines.
Decent Backup system - Don't use windows integrated backup, its better than it was but look towards backupassist or Ideally Veeam, you should also be testing your backups at least monthly, actually restoring a subset of important files and checking they open OK or testing your custom application if you use one (sage or other accounts software)
You will find an ML350 a good benchmark server or DL360/380 for rack. I would steer clear of NAS devices if your above 5+ users, whilst feature rich they generally have poor support and will cost you more in downtime than a well maintained server..
Support - Maintain hardware warranty on your server and have spare switches etc. I guess a large chunk of your costs are actually installation costs from your IT company. They are likely advising solutions they have worked with and tested, if you suddenly go against the grain you are going to be reducing their effectiveness in responding to support issues - again this is where your time lost £££ per hour comes in. If you have a special setup because you've gone cheap and cheerful and the vendor support in taiwan takes 2 weeks to respond to your issue, or you have no support at all - this is going to cost you money in the long run.
Saying they will support anything is probably incorrect - i'd get this checked over before you commit to anything and have a conversation with them, if anything goes wrong its down to you to fix if your speccing a quoting, this is what a proportion of your support costs come from - if they spec it they support and recommend it.
